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ScienceWeek
SCIENCEWEEK
ScienceWeek
May 30, 2003
Vol. 7 Number 22A
An Online Digest of Research in the Sciences
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In any field find the strangest thing and then explore it.
-- John Archibald Wheeler
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Section 1
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Symposium: High Energy Astrophysics
1. Introduction
2. Pulsars
3. Quasars and Microquasars
4. Ultra High Energy Cosmic Rays
5. Gamma Ray Bursts
6. Relativistic Jets
Notices and Subscription Information
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Section 2
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SYMPOSIUM: HIGH-ENERGY ASTROPHYSICS
1. INTRODUCTION
During the life of a star, two opposing forces control the star's
equilibrium: the gravitational force, which drives the collapse
of the star's mass inward to the center of gravity, and the
counteracting outward pressure derived from the nuclear fusion
reactions in the star's core. When the nuclear fuel burns out,
the star begins its death and gravitational collapse occurs. If
the terminal stages of star death (during which large amounts of
stellar material are blown away) leave a remnant star mass
greater than 3 solar masses, the ultimate gravitational collapse
will produce a black hole, a relativistic singularity. A black
hole is a localized region of space from which neither matter nor
radiation can escape. The "trapping" occurs because the requisite
escape velocity, which can be calculated from the relevant
equations, exceeds the velocity of light and is therefore
unattainable.
Another view of a black hole is that it is a mass that has
collapsed to such a small volume that its gravity prevents the
escape of all radiation. Space and time essentially have no
meaning in a black hole. The boundary of the black hole is called
the "event horizon", because any event within the boundary is
invisible outside, the invisibility resulting from the fact that
no radiation can escape to be detected. The radius of the black
hole depends upon how much matter has fallen into the region; it
is called the "Schwarzschild radius", and it is usually a few
kilometers. However, massive black holes are possible and are
thought to be the source of quasars (quasi-stellar objects),
which are extremely luminous sources radiating energy over the
entire spectrum from x-rays to radio waves, and which are
apparently the oldest and most distant objects in the universe.
If quasars indeed involve black holes, the radiation is from
material just outside the black hole, and not from anything
within it. Nothing inside a black hole can get out of it. Other
massive black holes, closer to us than quasars, are apparently
the centers of galaxies, both galaxies with active centers and
galaxies with dormant centers.
If, after the blow-off at star death, the remnant mass is between
1.4 and 2 to 3 solar-masses, the star will collapse into a
neutron star, a body with a radius of only 10 to 15 kilometers,
but with a core so dense that its component protons and electrons
have merged into neutrons. The average density of a neutron star
is 10^(15) grams per cubic centimeter and the weight of an object
on the surface of a neutron star would be 10^(11) times its
weight on the surface of the Earth. Neutron stars apparently have
an outer shell of iron, but it is iron like no Earth iron, an
iron of 4 orders of magnitude greater density. Theory predicts
that a neutron star should rotate very rapidly, be extremely hot,
and have an intense magnetic field. Pulsars, sources of pulsed
radio energy, are evidently spinning neutron stars which emit
beams of radiation from their magnetic poles.
ON QUASARS
Quasars (also called quasi-stellar objects, or QSOs) are small,
powerful, but extremely remote sources of radiation. Astronomers
now understand quasars as very distant, very powerful active
galaxies. Their significance, however, goes far beyond this
interpretation. They are so distant that their look-back times
are tremendous -- as much as 90 percent of the age of the
universe. Thus the discovery of quasars in the 1960s gave
astronomers an intriguing new view of the most distant, and the
youngest, parts of the universe.
Concerning the discovery of quasars, in the early 1960s, radio
interferometers showed that a number of radio sources are much
smaller than normal radio galaxies. Photographs of the location
of these radio sources did not reveal a central galaxy, not even
a faint wisp, but rather a single star-like point of light. The
first of these objects so identified was 3C 48, and later the
source 3C 273 was added. Though these objects emitted radio
signals like those from radio galaxies, they were obviously not
normal radio galaxies. Even the most distant photographable
galaxies look fuzzy, but these seemed like stars. Their spectra,
however, were totally unlike stellar spectra, so the objects were
called quasi-stellar objects.
Adapted from: Michael A. Seeds: Horizons: Exploring the Universe.
Wadsworth 1995, p.310.
ON PULSARS
Priority for the discovery of pulsars went to Anthony Hewish and
Jocelyn Bell, Cambridge radio astronomers, whose discovery of
"pulsars" constituted one of the most remarkable pieces of
serendipity in modern science.
Hewish built a special instrument with an important special
feature: it was sensitive enough to record rapid changes in the
intensity of the radiation from distant sources. And he found
what he was looking for: just as stars twinkle because their
light passes through turbulent air, so some radio sources
"scintillate" because the radio waves pass through an irregular
medium on the way toward us. But his research student, Jocelyn
Bell, found variations of a quite distinctive kind -- sporadic
series of regular pulses, each pulse lasting a fraction of a
second, coming from specific points in the sky. A frantic few
months of effort ensued. The Cambridge radio astronomers had to
check whether the signals had a terrestrial origin (maybe some
secret space project?). Three more of these mysterious sources
were soon found, each ticking at a well-defined rate. Could they
perhaps be signals from intelligent extraterrestrials? This idea
was never taken very seriously, but the sources were jocularly
referred to as LGM 1, 2, 3, and 4 (for "little green men").
When this discovery was announced in the journal Nature, even the
other astronomers in Cambridge were astonished. Hewish and his
colleagues had not shared their excitement with anyone outside a
tight-knit group. This concealment annoyed some of us at the
time, but in retrospect I think Hewish was no more than prudent.
Only a few months elapsed between Jocelyn Bell's first
intimations and the actual publication, so nobody's chance of
follow-up work was seriously delayed. And, for most of those
months, Hewish and Bell weren't completely confident that the
signals were "real". If the sporadic radio pulses had turned out
to have a mundane interpretation, or to arise from some fault in
their equipment, a premature announcement would not only have
been embarrassing, but might have wasted the efforts of many
other astronomers who would undoubtedly have followed up any
rumor of this kind.
What could these objects be? An ordinary star like the Sun would
fly apart if it pulsed or rotated much faster than once per hour.
Bodies that turned on and off in a fraction of a second plainly
had to be much more compact. Were they white dwarfs, or maybe
neutron stars? Were they pulsing or spinning? All these options
(and many others) had their advocates. The Cambridge group
originally favored pulsating white dwarf stars. (A naive inquirer
at a press conference was perplexed about how, at such a great
distance, white dwarfs could be distinguished from little green
men!)
The case for rotating neutron stars was first clearly argued by
Thomas Gold. There were good reasons for expecting neutron stars
to form when the cores of heavy stars collapsed, triggering
supernova explosions. They would be so small, and have such
strong gravity, that they could spin as fast as a thousand
revolutions per second without flying apart. The spin rate would
provide a natural stable clock; a "lighthouse beam" anchored to
the star would send an intense pulse toward us once per
revolution.
Only a year later, the debate was settled in Gold's favor. A very
fast pulsar was found at the center of the Crab Nebula,
transmitting 30 pulses per second: a white dwarf could neither
rotate nor pulsate as fast as that, but such rapid spin was no
problem for a neutron star. Moreover, careful timing showed that
the pulse rate was gradually slowing down: this was natural if
energy stored in the star's spin was being gradually converted
into radiation, and into a wind of particles that keep the Crab
Nebula shining in blue light.
Adapted from: Martin Rees: Before the Beginning: Our Universe and
Others. Perseus Books 1997, p.71.
ON PULSARS AND QUASARS
Some neutron stars, called "pulsars", produce an unequivocal "I
am a neutron star" cry: Their X-rays, or in some cases radio
waves, come in sharp pulses that are very precisely timed. The
timing is as precise, in some cases, as the ticking of our best
atomic clocks. Those pulses can only be explained as due to beams
of radiation shining off a neutron star's surface and swinging
past Earth as the star rotates -- the analogue of a rotating
light beacon at a rural airport or in a lighthouse. Why is this
the only possible explanation? Such precise timing can come only
from the rotation of a massive object with massive inertia and
thus massive resistance to erratic forces that would make the
timing erratic; of all the massive objects ever conceived by the
minds of astrophysicists, only neutron stars and black holes can
spin at the enormous rates (hundreds of rotations per second) of
some pulsars; and only neutron stars, not black holes, can
produce rotating beams, because black holes cannot have "hair".
(Any source of such a beam, attached to the hole's horizon, would
be an example of the type of "hair" that a black hole cannot hang
on to)...
The idea that gigantic black holes might power quasars and radio
galaxies was conceived by Edwin Salpeter and Yakov Borisovich
Zei'dovich in 1964... This idea was an obvious application of the
Salpeter-ZeI'dovich discovery that gas streams, falling toward a
black hole, should collide and radiate. A more complete and
realistic description of the fall of gas streams toward a black
hole was devised in 1969 by Donald Lynden-Bell, a British
astrophysicist in Cambridge. Lynden-Bell argued, convincingly,
that after the gas streams collide, they will join together, and
then centrifugal forces will make them spiral around and around
the hole many times before falling in; and as they spiral, they
will form a disk-shaped object, much like the rings around the
planet Saturn -- an "accretion disk", Lynden-Bell called it,
since the gas is "accreting" onto the hole. In the accretion
disk, adjacent gas streams will rub against each other, and
intense friction from that rubbing will heat the disk to high
temperatures... We normally think of friction as a poor source of
heat. Recall the unfortunate Boy Scout who tries to start a fire
by rubbing two sticks together! However, the Boy Scout is limited
by his meager muscle power, while an accretion disk's friction
feeds off gravitational energy. Since the gravitational energy is
enormous, far larger than nuclear energy, the friction is easily
up to the task of heating the disk and making it shine 100 times
more brightly than the most luminous galaxies.
Adapted from: Kip S. Thorne: Black Holes & Time Warps: Einstein's
Outrageous Legacy. W.W. Norton 1994, pp.317,346.
ON COSMIC RAYS
Radio, X-ray and gamma-ray astronomy have resulted in many
discoveries which can only be interpreted in terms of the
presence of large fluxes of relativistic particles in galaxies.
In parallel with these developments, cosmic ray studies opened up
new areas of astrophysical importance through direct observation
of high energy particles at the top of the atmosphere and in the
environment of the Earth from satellites and, for the very
highest energy cosmic rays, from the surface of the Earth by the
large air-shower arrays.
Cosmic radiation (what we would now call cosmic rays) was
discovered as long ago as 1912 by Victor Hess (1883-1964), but
the astrophysical understanding of the origin and propagation of
these particles had to await the 1960s when cosmic ray particle
detectors were flown in satellites. These observations
established many crucial facts about the particles detected in
the cosmic radiation. First of all, the energy spectra of the
particles are almost exactly the same as the typical spectrum of
high-energy particles inferred to be present in both Galactic and
extragalactic nonthermal radio sources. Observations indicate
that the cosmic ray particles observed at the top of the
atmosphere are only part of a population of high-energy particles
pervading the whole Galaxy.
Subsequent satellite observatories have determined the chemical
composition and detailed energy spectra of cosmic ray nuclei.
Remarkably, the chemical composition of the cosmic rays is
similar to the abundances of the elements in the Sun, although
there are some variations in the abundances at the higher
energies. These observations provide evidence on the chemical
composition of the cosmic rays as they left their sources and
also about the modifications which could have taken place during
propagation from their sources to the Earth. These observations
are very important for high energy astrophysics because they are
the only particles which we can detect which have traversed a
considerable distance through the interstellar medium and which
were accelerated in events such as supernovae and possibly
pulsars in the relatively recent past, probably within the last
10^(7) years.
At the very highest energies, cosmic rays are detected by large
air-shower arrays on the surface of the Earth. The arrival rate
of the most energetic particles is very low indeed, but particles
with energies up to about 10^(20) eV have been detected. One
important puzzle is the origin of these very high energy
particles. Their arrival directions seem to be reasonably
isotropic and, at these very high energies, these should not be
significantly influenced by the magnetic field in our own Galaxy.
Adapted from: Paul Davies (Ed.): The New Physics. Cambridge
University Press 1989, p.100
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2. PULSARS
ON PULSARS AND MAGNETARS
In astrophysics, the term "nova" refers to a class of exploding
stars whose luminosity temporarily increases from several
thousand to as much as 10^(5) times its normal level. Most novas
are thought to involve special double-star systems ("close
binaries"), one star a red giant and the other star a white
dwarf. If an expansion of the red giant encroaches the
gravitational domain of the white dwarf, the intense
gravitational field of the white dwarf pulls material from the
red giant, and this material accumulates on the surface of the
white dwarf until a nuclear explosion occurs.
The term "supernova" refers to a completely different phenomenon:
a supernova is any of a class of violently exploding stars whose
luminosity after eruption suddenly increases millions or billions
of times its normal level, the supernova explosion, unlike a nova
explosion, a cataclysmic event associated with the essential end
of the active (energy-generating) life of the star. In general,
the cause of the explosion is believed to be as follows:
a) When a star exhausts its nuclear fuel, the star undergoes
gravitational collapse, this collapse resulting in one of 3
possible objects, depending on the mass of the collapsing star:
black hole, neutron star, white dwarf. If the mass is greater
than approximately 3 solar-masses, a black hole will result; if
the mass if less than 3 but more than approximately 1.4 solar-
masses, a neutron star will result; is the mass is less than
approximately 1.4 solar-masses, a white dwarf star will result.
b) The core of a collapsing star of intermediate mass (1.4 to 3
solar-masses) soon consists almost entirely of neutrons, the core
with a diameter of only approximately 20 kilometers, and with a
mass equal to at least several solar-masses -- the result a core
of enormous density.
c) It is believed that a supernova explosion occurs when material
falling in from the outer layers of the star rebounds off the
dense core, which has stopped collapsing and now presents a hard
surface (iron of enormous density) to the infalling gases. The
shock wave generated by this collision propagates outward and
blows off the outer layers of the star. When a star "goes
supernova", material equaling the material of several Suns may be
blasted into space with enough energy so that the supernova
outshines its entire home galaxy.
The existence of neutron stars was first proposed by Lev Landau
(1908-1968) in 1932, and their relationship to supernovas was
first suggested by W. Baade (1893-1960) and F. Zwicky (1898-1974)
in 1934, who wrote the following famous sentence in a short
paper: "With all reserve we advance the view that a supernova
represents the transition of an ordinary star into a neutron star
consisting mainly of neutrons." It took 33 years for the first
apparent evidence of neutron stars to be obtained -- the objects
called "pulsars" -- a discovery made by A. Hewish and J. Bell in
1967 [see *Note #1 on (Susan) Jocelyn Bell]. Meanwhile, in 1939,
J. Robert Oppenheimer (1904-1967) and others developed the idea
of neutron star production in the cores of supernovas into an
important theory of stellar evolution.
A "pulsar" (pulsating radio star) is any of a class of cosmic
objects that emit extremely regular pulses of radio waves, with
several such objects known to emit pulses of visible light, x-
rays, and gamma-rays as well. In general, pulsars are believed to
be rapidly rotating neutron stars. It is believed that neutrons
at the surface of the neutron star decay into protons and
electrons, and as these charged particles are released from the
surface, they enter an intense magnetic field surrounding the
star and rotate with the star. The particles accelerate to speeds
approaching the velocity of light, and the particles give off
electromagnetic radiation by "synchrotron emission" -- the
electromagnetic radiation emitted by charged particles moving in
a magnetic field at a velocity close to that of light. This
radiation is released as intense beams from the magnetic poles of
the pulsar, and since the magnetic poles do not coincide with the
rotational poles, the emitted radiation beam is rotated and
sweeps regularly past the Earth with each complete rotation (like
the rotating beam of a light-house), the result an evenly-spaced
series of pulses detected by ground-based telescopes. At present,
approximately 600 pulsars have been identified.
On August 27, 1998, a tremendous burst of gamma-rays and x-rays,
the burst lasting approximately 5 minutes, impacted the Earth,
the burst powerful enough to produce noticeable ionization of the
Earth's atmosphere. The x-rays were found to vary with a 5.16
second period, precisely the same as that of a known active x-ray
source in a galaxy 20,000 light-years from Earth in the
constellation Aquila. Such x-ray sources are believed to be
highly magnetic rotating neutron stars, and it was suggested that
the burst was caused by a "starquake" on a neutron star with an
intense magnetic field possibly 10^(15) times larger than that of
Earth. Such stellar objects were named "magnetars", and one
proposal is that a magnetar's enormous magnetic field
occasionally cracks open the crust of the star, and this leads in
some way to the production of energetic charged particles and
gamma-rays.
Jim Cordes (Cornell University, US) presents a commentary on some
recent research on pulsars, the author making the following
points concerning pulsars and magnetars:
1) The author points out that most neutron stars are detected as
"ordinary" radio pulsars with magnetic fields of 10^(12) gauss --
10^(12) times as strong as the magnetic field of Earth -- and
spin periods between 16 milliseconds and 8.5 seconds. The rapid
rotation combines with the strong magnetic field to produce
electric forces that generate particles moving near the speed of
light. These relativistic particles radiate intense
electromagnetic waves directed along the magnetic poles of the
neutron star, which appear to an observer as pulses of radiation
as the star rotates. The magnetic field also slows the rotation
of the pulsar down through "magnetic braking", an effect caused
by the radiation carrying away the angular momentum of the star.
Ordinary pulsars remain radio "loud" for approximately 10 million
years, the time it takes for the pulsar to slow down to a spin
rate at which particle creation stops.
2) During the past 5 years, considerable interest has focused on
objects that appear to be even more highly magnetized than
typical radio pulsars. These are the so-called "magnetars",
objects with magnetic fields that range from approximately
10^(13) to 10^(15) gauss. A few magnetars have been identified in
x-ray and gamma-ray observations. Magnetars spin down much more
rapidly than radio pulsars, on timescales of 10,000 years.
3) A combination of data from two x-ray satellites recently led
to the discovery of a very young pulsar with an unusually high
magnetic field (E. Gotthelf et al: Astrophys. J. Lett. 542:37
2000). This x-ray pulsar is associated with the supernova remnant
Kesteven 75. Although neutron stars are thought to be created in
supernova explosions, there are surprisingly few clear examples
of this. Gotthelf et al suggest that the unusual x-ray pulsar
observed by them may be a missing link between different classes
of pulsars, and that this new pulsar could provide important
clues to understanding how particles are created and radiate in
the magnetospheres surrounding neutron stars. Cordes states: "It
may turn out that many of the neutron stars in our Galaxy are
born with properties similar to this pulsar rather than to the
bulk of previously known neutron stars."
Nature 2001 409:296
Notes:
*Note #1: It is generally agreed that [Susan] Jocelyn Bell, who
was 24 years old and Hewish's graduate student at the time
(1967), made the actual discovery of the first pulsar by noticing
unexplained pulses in radio telescope data contained in 100-foot
lengths per day of paper charts, and that her discovery was
instrumental in Hewish winning his Nobel Prize in Physics in
1974, which he shared with Martin Ryle (1918-1984), a prime
figure in the development of radio-telescope astronomy. The Bell
discovery was made while Bell, Hewish, and Ryle were at Cambridge
University (UK), and the astronomer Martin Rees, who was of the
faculty at Cambridge at that time, writes of Jocelyn Bell as
follows: "Jocelyn Bell received less than her fair share of
credit for the discovery of pulsars. This happened, I think,
because of the social pressures which (then even more than now)
impeded women's careers, and lowered their scientific
aspirations. After getting her PhD, Jocelyn Bell left active
research for several years -- giving priority to her husband's
career seemed at that time the 'natural' thing to do. Had she
instead continued, and acquired 'visibility' by joining the small
cohort of radio astronomers who, over the next few years [after
1967] consolidated our knowledge of pulsars and discovered many
more -- as, almost certainly, a _man_ with her extraordinary
initial record would have done -- it is hard to believe her
achievements would have been slighted to the same extent."
[Martin Rees: _Before the Beginning_, (1997) p.263]. It has been
suggested that in an earlier age CP-1919, the first observed
pulsar, would have been called "Bell's Star". No matter the name
of the first observed pulsar, it was discovered by Susan Jocelyn
Bell (now Susan Jocelyn Bell Burnell), and there are many who
believe that the Nobel Prize in Physics of 1974 should read Ryle,
Hewish, and Bell. Bell Burnell, however, disagrees, and she has
stated: "Nobel prizes are based on long-standing research, not on
flash-in-the-pan observation by a research student. The award to
me would have debased the prize."
Related Background:
AN X-RAY NEBULA ASSOCIATED WITH THE MILLISECOND PULSAR B1957+20
The following points are made by B. Stappers et al (Stichting
ASTRON, NL):
1) Millisecond pulsars are old neutron stars (typically ~3
billion years old) that have been spun up to a rapid rotation
rate (25 ms) by accretion of material from a binary companion
(1,2). After the accretion phase, they appear as radio pulsars
with surface magnetic field strengths of ~10^(8) G, which,
combined with their older ages and rapid rotation rates, means
that they form a separate population from younger pulsars.
2) PSR B1957+20 is the second fastest spinning pulsar known. The
pulsar is in a 9.16-hour binary orbit with a low-mass companion
star. The wind of the companion star eclipses the radio emission
for ~10% of every orbit. The PSR B1957+20 binary system provides
an excellent opportunity to study the wind of a weakly
magnetized, recycled neutron star. The wind is ablating and may
eventually evaporate the low-mass companion star. Ablation and
heating of the companion star (5) are believed to be caused by x-
or gamma-rays generated in an intrabinary shock between the
pulsar wind and that of the companion star. Meanwhile, the high
space velocity of the pulsar, as it moves through the
interstellar medium, generates sufficient ram pressure to confine
the pulsar wind and results in the formation of a bow shock.
3) In summary: The authors report they have detected an x-ray
nebula around the binary millisecond pulsar B1957+20. A narrow
tail, corresponding to the shocked pulsar wind, is seen interior
to the known H bow shock and proves the long-held assumption that
the rotational energy of millisecond pulsars is dissipated
through relativistic winds. Unresolved x-ray emission likely
represents the shock where the winds of the pulsar and its
companion collide. This emission indicates that the efficiency
with which relativistic particles are accelerated in the
postshock flow is similar to that for young pulsars, despite the
shock proximity and much weaker surface magnetic field of this
millisecond pulsar.
References (abridged):
1. M. A. Alpar, A. F. Cheng, M. A. Ruderman, J. Shaham, Nature
300, 728 (1982)
2. D. Bhattacharya and E. P. J. van den Heuvel, Phys. Rep. 203, 1
(1991)
3. A. S. Fruchter, D. R. Stinebring, J. H. Taylor, Nature 333,
237 (1988)
4. M. Toscano, et al., Mon. Not. R. Astron. Soc. 307, 925 (1999)
5. A. S. Fruchter, J. Bookbinder, C. Bailyn, Astrophys. J. 443,
L21 (1995)
Science 2003 299:1372
Related Background:
MAGNETAR-LIKE X-RAY BURSTS FROM AN ANOMALOUS X-RAY PULSAR
The following points are made by F.P. Gavriil et al (McGill
University, CA):
1) Anomalous X-ray pulsars (AXPs) are a class of rare X-ray
emitting pulsars whose energy source has been perplexing for some
20 years(1-3). Unlike other X-ray emitting pulsars, AXPs cannot
be powered by rotational energy or by accretion of matter from a
binary companion star, hence the designation "anomalous". Many of
the rotational and radiative properties of the AXPs are
strikingly similar to those of another class of exotic objects,
the soft-gamma-ray repeaters (SGRs). But the defining property of
the SGRs -- their low-energy-gamma-ray and X-ray bursts -- has
not hitherto been observed for AXPs. Soft-gamma-ray repeaters are
thought to be "magnetars", which are young neutron stars whose
emission is powered by the decay of an ultra-high magnetic
field(4,5); the suggestion that AXPs might also be magnetars has
been controversial.
2) SGRs are believed to be magnetars because the high magnetic
field provides the torque for their rapid spin-down, as well as
the energy to power their bursts and quiescent X-ray emission(4).
AXPs have been suggested to be magnetars, albeit less active,
because of their similar spin periods, rates of spin-down and
location in the Galactic plane, and their similar (though
somewhat softer) X-ray spectra to those of SGRs in quiescence.
The physical difference between the two classes is unknown, but,
in the magnetar model, is probably related to the magnitude or
distribution of the stellar magnetic field. However, the apparent
absence of any bursting behavior in AXPs has led to suggestions
that they could be powered, not by magnetism, but by accretion
from a disk of material remaining after the birth supernova
event. If so, the observational similarities between AXPs and
SGRs must be purely coincidental.
3) The authors report two X-ray bursts, with properties similar
to those of SGRs, from the direction of an anomalous X-ray
pulsar. The authors suggest these events imply a close
relationship (perhaps evolutionary) between AXPs and SGRs, with
both being magnetars.
References (abridged):
1. Fahlman, G. G. & Gregory, P. C. An X-ray pulsar in SNR G109.1-
1.0. Nature 293, 202-204 (1981)
2. van Paradijs, J., Taam, R. E. & van den Heuvel, E. P. J. On
the nature of the "anomalous" 6-s X-ray pulsars. Astron.
Astrophys. 299, L41-L44 (1995)
3. Mereghetti, S. & Stella, L. The very low mass X-ray binary
pulsars: A new class of sources? Astrophys. J. 442, L17-L20
(1995)
4. Thompson, C. & Duncan, R. C. The soft gamma repeaters as very
strongly magnetized neutron stars--I. Radiative mechanism for
outbursts. Mon. Not. R. Astron. Soc. 275, 255-300 (1995)
5. Kouveliotou, C. et al. An X-ray pulsar with a superstrong
magnetic field in the soft gamma-ray repeater SGR 1806-20. Nature
393, 235-237 (1998)
Nature 2002 419:142
Related Background:
OPTICAL PULSATIONS FROM THE ANOMALOUS X-RAY PULSAR 4U0142+61
The following points are made by B. Kern and C. Martin
(California Institute of Technology, US):
1) Anomalous X-ray pulsars(1) (AXPs) differ from ordinary radio
pulsars in that their X-ray luminosity is orders of magnitude
greater than their rate of rotational energy loss, and so they
require an additional energy source. One possibility is that AXPs
are highly magnetized neuron stars(2) -- or "magnetars" -- having
surface magnetic fields greater than 10^(14) G. This would make
them similar to the soft gamma-ray repeaters (SGRs)(3), but
alternative models that do not require extreme magnetic fields
also exist.
2) An optical counterpart to the AXP 4U0142+61 was recently
discovered(4), consistent with emission from a magnetar, but also
from a magnetized hot white dwarf(5), or an accreting isolated
neutron star.
3) The authors report the detection of optical pulsations from
4U0142+61. The pulsed fraction of optical light (27 per cent) is
five to ten times greater than that of soft X-rays, from which
the authors conclude that 4U0142+61 is a magnetar. Although this
establishes a direct relationship between AXPs and the soft
gamma-ray repeaters, the evolutionary connection between AXPs,
SGRs and radio pulsars remains controversial.
References (abridged):
1. Mereghetti, S. in The Neutron Star-Black Hole Connection (eds
Kouveliotou, C., Ventura, J. & van den Heuvel, E. P. J.) 351-368
(Kluwer Academic, Dordrecht, 2001); also preprint astro-
ph/9911252 at http://xxx.lanl.gov/ (1999)
2. Duncan, R. C. & Thompson, C. Formation of very strongly
magnetized neutron stars: Implications for gamma-ray bursts.
Astrophys. J. 392, L9-L13 (1992)
3. Kouveliotou, C. et al. An X-ray pulsar with a superstrong
magnetic field in the soft -ray repeater SGR 1806-20. Nature 393,
235-237 (1998)
4. Hulleman, F., van Kerkwijk, M. H. & Kulkarni, S. R. An optical
counterpart to the anomalous X-ray pulsar 4U0142+61. Nature 408,
689-692 (2000)
5. Paczyski, B. X-ray pulsar 1E 2259 + 586: A merged white dwarf
with a 7 second rotation period? Astrophys. J. 365, L9-L12 (1990)
Nature 2002 417:527
Related Background:
NANOSECOND RADIO BURSTS FROM STRONG PLASMA TURBULENCE IN THE CRAB
PULSAR
The following points are made by T.H. Hankins et al (New Mexico
Tech Socorro, US):
1) The Crab pulsar was discovered(1) by the occasional
exceptionally bright radio pulses it emits, subsequently dubbed
"giant" pulses. Only two other pulsars are known to emit giant
pulses(2,3). There is no satisfactory explanation for the
occurrence of giant pulses, nor is there a complete theory of the
pulsar emission mechanism in general. Competing models for the
radio emission mechanism can be distinguished by the temporal
structure of their coherent emission.
2) The authors report the discovery of isolated, highly
polarized, two-nanosecond subpulses within the giant radio pulses
from the Crab pulsar. The plasma structures responsible for these
emissions must be smaller than one meter in size, making them by
far the smallest objects ever detected and resolved outside the
Solar System, and the brightest transient radio sources in the
sky. Only one of the current models -- the collapse of plasma-
turbulent wave packets in the pulsar magnetosphere -- can account
for these nanopulses.(4,5)
References (abridged):
1. Staelin, D. H. & Reifenstein, E. C. III Pulsating radio
sources near the Crab nebula. Science 162, 1481-1483 (1968)
2. Cognard, I., Shrauner, J. A., Taylor, J. H. & Thorsett, S. E.
Giant radio pulses from a millisecond pulsar. Astrophys J. 457,
L81-L84 (1996)
3. Romani, R. W. & Johnston, S. Giant pulses from the millisecond
pulsar B1821-24. Astrophys J. 557, L93-L96 (2001)
4. Hankins, T. H. Microsecond intensity variations in the radio
emissions from CP 0950. Astrophys J. 169, 487-491 (1971)
5. Hankins, T. H. & Rickett, B. J. Pulsar signal processing.
Meth. Comp. Phys. 14, 55-129 (Academic, New York, 1975)
Nature 2003 433:141
Related Background:
xON THE AGES OF PULSARS
B.M. Gaensler and D.A. Frail (2 installations, US) present data
concerning the age of a particular pulsar (B1757-24) determined
by analysis of its motions. The authors make the following
points:
1) The assumption that the characteristic age of a pulsar is
approximately its true age has led to some puzzling results,
including many pulsars with small characteristic ages having no
associated supernova remnants. The properties of the pulsar
B1757-24, which is located just outside the edge of a supernova
remnant, indicate that the pulsar was born at the center of the
remnant with a substantial velocity, and that it has subsequently
overtaken the expanding blast wave. With a characteristic age of
16,000 years, this pulsar is expected to have a proper motion of
63 to 80 milliarcseconds (mas) per year.
2) The authors report observations of the nebula surrounding
pulsar B1757-24, and the observations limit the proper motion of
this pulsar to 25 milliarcseconds per year. This implies a
minimum age of 39,000 years. A more detailed analysis suggests
the true age of this pulsar may be 170,000 years, which is
significantly larger than the characteristic age. The authors
suggest from this result and from other discrepancies associated
with pulsars, that characteristic ages greatly underestimate the
true ages of pulsars.
3) The authors conclude: "If other pulsars are indeed older than
they seem, our understanding of pulsar velocities, asymmetries in
supernova explosions, the fraction of supernovae that produce
pulsars, and the physics of neutron star structure and cooling
must be reconsidered."
4) In a commentary on the above work, John H. Seiradakis
(University of Thessaloniki, GR) states: "This discrepancy
between the characteristic age of the pulsar [B-1757-24] and the
age of the supernova remnant poses a serious problem to either
the association between the pulsar and the supernova remnant or
[to] the common belief that the characteristic age of pulsars
represents their true age."
Nature 13 Jul 00 406:139,158
A RADIO PULSAR THAT CHALLENGES EMISSION MODELS
Theory predicts that a neutron star should rotate very rapidly,
be extremely hot, and have an intense magnetic field. "Radio
pulsars" are apparently rotating neutron stars that emit beams of
radio waves from regions above their magnetic poles, with the
radio emission arising from the acceleration of charged particles
above the magnetic poles. As the neutron star rotates, a beam of
radio waves sweeps across the Earth and a radio pulse is
observed, much like the beam from a lighthouse. The pulse periods
of neutron stars can be measured to an accuracy of approximately
1 part in 10^(10). Current theories of the emission mechanism
require continuous electron-*positron pair production, with the
potential responsible for accelerating the particles inversely
related to the spin period. According to theory, production of
electron-positron pairs will cease when the potential drops below
a threshold, and thus the models predict that radio emission will
cease when the pulsation period exceeds a value that depends on
the strength and configuration of the magnetic field. For a
number of years, this general scheme has been the consensus view
concerning pulsars, but new evidence has now arrived which is in
apparent serious conflict with current ideas concerning pulsar
dynamics.
M.D. Young et al (3 authors at 3 installations, AU) now report
that the pulsar PSR J2144-3933, previously thought to have a
period of 2.84 seconds, actually has a period of 8.51 seconds,
which is by far the longest period of any known radio pulsar. The
authors point out that under the usual model assumptions this
slowly rotating pulsar should not be emitting a radio beam. The
authors suggest that either the model assumptions are wrong, or
current theories of radio emission must be revised. The authors
further point out that consideration of the luminosity parameters
of PSR J2144-3933 imply that we can observe only a very small
proportion of the total population of such objects in the Galaxy.
The authors conclude: "While extrapolation from the detection of
a single object is always uncertain (some would say foolhardy),
there is no reason to suppose that PSR J2144-3933 is unique. With
this caveat, this detection implies a Galactic population of
similar pulsars of the order of 10^(5), comparable to previous
estimates of the size of the total pulsar population [in our
Galaxy]."
Nature 1999 400:848
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3. QUASARS AND MICROQUASARS
ON MICROQUASARS
The following points are made by Michael P. Rupen (National Radio
Astronomy Observatory Socorro, US):
1) Over the past decade, astronomers have discovered several
transient, highly relativistic jet sources associated with
stellar-mass black hole binary systems in our galaxy (1). These
are called "microquasars", by analogy to the much more powerful
relativistic jets found in radio galaxies at moderate distances,
and to quasars at the edge of the known Universe.
2) In radio galaxies and quasars, the jets are powered by
accretion onto massive black holes. They are surprisingly
efficient at transporting the resulting energy out to large
distances, often well beyond the edge of the host galaxy (2,3).
There the jet may collide with the intergalactic medium,
releasing its bulk kinetic energy in a terminal shock, from which
the material flows back to inflate huge radio lobes.
3) The jets in microquasars seem very similar in terms of speed,
power source, and collimation. One would therefore expect similar
fireworks when they hit the interstellar medium. A few unusually
steady and long-lived sources (4) show signs of such collisions,
but most microquasars do not; in fact, there is little evidence
that these short-lived jets interact at all with their
surroundings. This is not too disturbing for any individual
source: Some jets remain collimated but fade as they move out,
and others may expand and get lost in the background. But some
microquasars must surely interact with the interstellar medium at
some point. Corbel et al. (5) report evidence that they do.
References (abridged):
1. L. F. Rodríguez, I. F. Mirabel, Annu. Rev. Astron. Astrophys.
37, 409 (1999)
2. A. H. Bridle, R. A. Perley, Annu. Rev. Astron. Astrophys. 22,
319 (1984)
3. M. C. Begelman, R. D. Blandford, M. J. Rees, Rev. Mod. Phys.
56, 255 (1984)
4. The most famous is SS433, where the jets clearly interact with
the remnant W50 some 80 light years away. But whereas the jets of
most microquasars are present only occasionally and for a brief
time (days to weeks), those in SS433 seem to have been "on"
almost continuously for thousands of years. The question is what
happens to the energy released in the more common, transient
events.
5. S. Corbel et al., Science 298, 196 (2002)
Science 2002 298:73.
Related Background:
INTERSTELLAR SCINTILLATION AS THE ORIGIN OF THE RAPID RADIO
VARIABILITY OF THE QUASAR J1819+3845
The following points are made by J. Dennett-Thorpe and A. De
Bruyn (University of Groningen, NL):
1) The liberation of gravitational energy as matter falls onto a
supermassive black hole at the center of a galaxy is believed to
explain the high luminosity of quasars.(1,2) The variability of
this emission from quasars and other types of active galactic
nuclei can provide information on the size of the emitting
regions and the physical process of fuelling the black hole. Some
active galactic nuclei are variable at optical (and shorter)
wavelengths, and display radio outbursts over years and decades.
These active galactic nuclei often also show faster intraday
variability at radio wavelengths(3,4). The origin of this rapid
variability has been extensively debated(5), but a correlation
between optical and radio variations in some sources suggests
that both are intrinsic. This would, however, require radiation
brightness temperatures that seem physically implausible, leading
to the suggestion that the rapid variations are caused by
scattering of the emission by the interstellar medium inside our
Galaxy.
2) The most variable extragalactic source known at radio
wavelengths is the quasar J1819+3845. This quasar shows
variations of factors of 4 or more on a timescale of hours,
easiest interpreted as scintillation due to scattering in the
interstellar medium. One can think of this scattering medium as
focusing and defocusing the waves from the source, producing a
pattern of dark and bright patches. As we move through these
patches we observe a temporal variation in intensity. In order to
display this effect, the source must be small; accordingly
J1819+3845 is only tens of microarcseconds in angular size (a few
light months at the source redshift 0.54).
3) The authors demonstrate that the rapid variations in the
extreme case of quasar J1819+3845 indeed arise from interstellar
scintillation. The transverse velocity of the scattering material
reveals the presence of plasma with a surprisingly high velocity
close to the Solar System.
References (abridged):
1. Rees, M. J. Black hole models for active galactic nuclei.
Annu. Rev. Astron. Astrophys. 22, 471-506 (1984)
2. Aller, H. D., Aller, M. F., Latimer, G. E. & Hodge, P. E.
Spectra and linear polarizations of extragalactic variable
sources at centimeter wavelengths. Astrophys. J. Suppl. Ser. 59,
513-768 (1985)
3. Heeschen, D. S. Flickering of extragalactic radio sources.
Astron. J. 89, 1111-1123 (1984)
4. Quirrenbach, A., Witzel, A., Krichbaum, T., Hummel, C. A. &
Alberdi, A. Rapid variability of extragalactic radio sources.
Nature 337, 442-444 (1989)
5. Wagner, S. J. & Witzel, A. Intraday variability in quasars and
BL Lac objects. Annu. Rev. Astron. Astrophys. 33, 163-198 (1995)
Nature 2002 415:57
Related Background:
SPECTRAL SIGNATURE OF COSMOLOGICAL INFALL OF GAS AROUND THE FIRST
QUASARS
The following points are made by Rennan Barkana and Abraham Loeb
(Tel Aviv University, IL):
1) Recent observations have shown that only a billion years after
the Big Bang the Universe was already lit up by bright quasars(1)
fuelled by the infall of gas onto supermassive black holes. The
masses of these early black holes are inferred from their
luminosities to be >10^(9) solar masses (M), which is a difficult
theoretical challenge to explain. Like nearby quasars, the early
objects could have formed in the central cores of massive host
galaxies. The formation of these hosts could be explained if,
like local large galaxies, they were assembled gravitationally
inside massive (> 10^(12) M) haloes of dark matter(2). There has
hitherto been no observational evidence for the presence of these
massive hosts or their surrounding haloes.
2) The authors demonstrate that the cosmic gas surrounding each
halo must respond to its strong gravitational pull, where
absorption by the infalling hydrogen produces a distinct spectral
signature. That signature can be seen in recent data(3,4).
3) The authors suggest their models provide direct evidence that
two characteristic properties of quasars at low redshift are also
applicable to bright quasars in the early Universe. These
properties include the quasar spectral template, which determines
the ionizing intensity of the quasar, and the relation between
black hole mass and halo velocity dispersion, which the authors
have used to determine the host halo mass. Both observed spectra
show a blue peak of about 75% of the height of the red (positive
velocity) peak, and this is roughly matched by the models.(5)
References (abridged):
1. Fan, X. et al. Survey of z > 5.8 quasars in the Sloan digital
sky survey. I. Discovery of three new quasars and the spatial
density of luminous quasars at z 6. Astron. J. 122, 2833-2849
(2001)
2. Barkana, R. & Loeb, A. In the beginning: the first sources of
light and the reionization of the universe. Phys. Rep. 349, 125-
238 (2001)
3. Zheng, W. et al. Five high-redshift quasars discovered in
commissioning imaging data of the Sloan Digital Sky Survey.
Astron. J. 120, 1607-1611 (2000)
4. Becker, R. H. et al. Evidence for reionization at z 6:
Detection of a Gunn-Peterson trough in a z = 6.28 quasar. Astron.
J. 122, 2850-2857 (2001)
5. Loeb, A. & Eisenstein, D. J. Probing early clustering with Ly
absorption lines beyond the quasar redshift. Astrophys. J. 448,
17-26 (1995)
Nature 2003 421:341
Related Background:
MAGNIFICATION OF LIGHT FROM MANY DISTANT QUASARS BY GRAVITATIONAL
LENSES
The following points are made by J. Stuart et al (Harvard
University, US):
1) Exceptionally bright quasars with redshifts up to z = 6.28
have recently been discovered(1). Quasars are thought to be
powered by the accretion of gas onto supermassive black holes at
the centers of galaxies. Their maximum (Eddington) luminosity
depends on the mass of the black hole, and the brighter quasars
are inferred to have black holes with masses of more than a few
billion solar masses. The existence of such massive black holes
poses a challenge to models for the formation of structures in
the early Universe(2,3), as it requires their formation within
one billion years of the Big Bang.
2) The authors demonstrate that up to one-third of known quasars
with z >= 6 will have had their observed flux magnified by a
factor of ten or more, as a consequence of gravitational lensing
by galaxies along the line of sight. The authors suggest that the
inferred abundance of quasar host galaxies, as well as the
luminosity density provided by the quasars, has therefore been
substantially overestimated.(4,5)
References (abridged):
1. Fan, X. et al. Survey of z > 5.8 quasars in the Sloan digital
sky survey. I. Discovery of three new quasars and the spatial
density of luminous quasars at z6. Astron. J. 122, 2833-2849
(2001)
2. Turner, E. L. Quasars and galaxy formation. I--The Z greater
than 4 objects. Astron. J. 101, 5-17 (1991)
3. Haiman, Z. & Loeb, A. What is the highest plausible redshift
for quasars? Astrophys. J. 503, 505-517 (2001)
4. Djorgovski, S. G., Castro, S., Stern, D. & Mahabal, A. A. On
the threshold of the reionization epoch. Astrophys. J. 560, L5-L8
(2001)
5. Press, W. H. & Schechter, P. Formation of galaxies and
clusters of galaxies by self-similar gravitational condensation.
Astrophys. J. 187, 425-438 (1974)
Nature 2002 417:923
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4. ULTRA HIGH ENERGY COSMIC RAYS
ULTRAHIGH-ENERGY COSMIC RAYS: PHYSICS AND ASTROPHYSICS AT EXTREME
ENERGIES
The following points are made by Günter Sigl (Institut
d'Astrophysique de Paris, FR):
1) High energy cosmic ray (CR) particles are shielded by Earth's
atmosphere and reveal their existence on the ground only by
indirect effects such as ionization and showers of secondary
charged particles covering areas up to many km^(2) for the
highest energy particles. Indeed, in 1912 Victor Hess discovered
CRs by measuring ionization from a balloon (1), and in 1938
Pierre Auger proved the existence of extensive air showers (EASs)
caused by primary particles with energies above 10^(15) eV by
simultaneously observing the arrival of secondary particles in
ground detectors many meters apart (2).
2) After almost 90 years of research, the origin of CRs is still
an open question, with a degree of uncertainty increasing with CR
energy (3): Only below 100 MeV kinetic energy, where the solar
wind shields protons coming from outside the solar system, must
the Sun give rise to the observed proton flux. Above that energy
the CR spectrum exhibits little structure and is approximated by
broken power laws. The bulk of the CRs up to at least that energy
are believed to originate within the Milky Way galaxy. At the
highest energies there is no apparent end to the CR spectrum, and
over the last few years giant air showers from CR primaries with
energies exceeding 10^(20) eV (4,5) have been detected. This
represents up to 50 J in what appears to be one elementary
particle, about 10^(8) times higher than energies achievable in
accelerator laboratories. The nature and origin of ultrahigh-
energy cosmic rays, and especially the ones above 10^(20) eV, are
mysterious.
3) In summary: The origin of cosmic rays is one of the major
unresolved questions in astrophysics. In particular, the highest
energy cosmic rays observed have macroscopic energies up to
several 10^(20) electron volts and thus provide a probe of
physics and astrophysics at energies unattained in laboratory
experiments. Theoretical explanations range from astrophysical
acceleration of charged particles, to particle physics beyond the
established standard model, and processes taking place at the
earliest moments of our universe. Distinguishing between these
scenarios requires detectors with effective areas in the 1000-
square-kilometer range, which are now under construction or in
the planning stage. Close connections between gamma-ray and
neutrino astrophysics add to the interdisciplinary character of
this field.
References (abridged):
1. V. F. Hess, Phys. Z. 13, 1084 (1912)
2. P. Auger, R. Maze, T. Grivet-Meyer, Acad. Sci. 206, 1721
(1938); P. Auger and R. Maze, Acad. Sci. 207, 228 (1938)
3. For a general introduction on cosmic rays see, e.g., V. S.
Berezinsky, S. V. Bulanov, V. A. Dogiel, V. L. Ginzburg, V. S.
Ptuskin, Astrophysics of Cosmic Rays (North-Holland, Amsterdam,
1990); T. K. Gaisser, Cosmic Rays and Particle Physics (Cambridge
Univ. Press, Cambridge, 1998)
4. M. Nagano and A. A. Watson, Rev. Mod. Phys. 72, 689 (2000)
5. For a summary of the data situation and experimental issues
see, e.g., S. Yoshida and H. Dai, J. Phys. G 24, 905 (1998); X.
Bertou, M. Boratav, A. Letessier-Selvon, Int. J. Mod. Phys. A15,
2181 (2000)
Science 2001 291:73
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5. GAMMA RAY BURSTS
GAMMA-RAY BURSTS: ACCUMULATING AFTERGLOW IMPLICATIONS, PROGENITOR
CLUES, AND PROSPECTS
The following points are made by P. Meszaros (Pennsylvania State
University, US):
1) Until a few years ago, gamma-ray bursts (GRBs) were known
predominantly as bursts of gamma-rays, largely devoid of any
observable traces at any other wavelengths. However, a striking
development in the last several years has been the measurement
and localization of fading x-ray signals from some GRBs, lasting
typically for days and making possible the optical and radio
detection of afterglows, which, as fading beacons, mark the
location of the fiery and brief GRB event. These afterglows in
turn enabled the measurement of redshift distances, the
identification of host galaxies, and the confirmation that GRBs
were, as suspected, at cosmological distances on the order of
billions of light years, similar to those of the most distant
galaxies and quasars. Even at those distances, they appear so
bright that their energy output must be on the order of 10^(51)
to 10^(54) erg/s, larger than that of any other type of source.
It is comparable to burning up the entire mass-energy of the sun
in a few tens of seconds, or to emit over that same period of
time as much energy as our entire Milky Way does in a hundred
years.
2) GRBs were first reported in 1973 on the basis of 1969-1971
observations by the Vela military satellites monitoring for
nuclear explosions in verification of the Nuclear Test Ban
Treaty. When these mysterious gamma-ray flashes, which did not
come from Earth's direction, were initially detected, the first
suspicion (quickly abandoned) was that they might be the product
of an advanced extraterrestrial civilization. Soon, however, it
was realized that this was a new and extremely puzzling cosmic
phenomenon. For the next 20 years, hundreds of GRB detections
were made, frustrating researchers as they continued to vanish
too soon to allow an accurate angular position to permit any
follow-up observations. The reason for this is that gamma-rays
are notoriously hard to focus, so gamma-ray images are generally
not very sharp.
3) In summary: Gamma-ray bursts (GRBs) are sudden, intense
flashes of gamma rays that, for a few blinding seconds, light up
in an otherwise fairly dark gamma-ray sky. They are detected at
the rate of about once a day, and while they are on, they
outshine every other gamma-ray source in the sky, including the
Sun. Major advances have been made in the last 3 or 4 years,
including the discovery of slowly fading x-ray, optical, and
radio afterglows of GRBs, the identification of host galaxies at
cosmological distances, and evidence showing that many GRBs are
associated with star-forming regions and possibly supernovae.
Progress has been made in understanding how the GRB and afterglow
radiation arises in terms of a relativistic fireball shock model.
These advances have opened new vistas and questions on the nature
of the central engine, the identity of their progenitors, the
effects of the environment, and their possible gravitational
wave, cosmic-ray, and neutrino luminosity. The debates on these
issues indicate that GRBs remain among the most mysterious
puzzles in astrophysics.(1-5)
References (abridged):
1. G. Fishman and C. Meegan, Annu. Rev. Astron. Astrophys. 33,
415 (1995)
2. D. Band, et al., Astrophys. J. 413, 281 (1993)
3. K. Hurley, et al., Nature 372, 652 (1994)
4. C. Kouveliotou, et al., Astrophys. J. 413, L101 (1993)
5. E. Fenimore, E. Ramirez-Ruiz, B. Wu, Astrophys. J. 518, L73
(1999)
Science 2001 291:65
Related Background
REDUCED ESTIMATES OF GAMMA-RAY BURST ENERGY
The following points are made by Tsvi Piran (Hebrew University
Jerusalem, US):
1) Several times a day, a short burst of gamma rays (GRB) reaches
Earth from outer space. The high-energy bursts last a few seconds
and arrive from random directions in the sky. Because of their
short duration, the exact location of the emitting sources could
not be pinned down until the BeppoSAX satellite discovered in
1997 that GRBs are followed by an x-ray afterglow lasting several
days. The exact positions given by the satellite enabled optical
and radio astronomers to detect optical and radio afterglows
lasting days to months. The host galaxies could be identified
once the afterglows had faded.
2) Redshift measurements of the host galaxies revealed that GRBs
are associated with an enormous energy output. In one extreme
case, GRB990123, an energy output of more than 10^(54) ergs was
estimated, comparable to the rest mass energy of a star. An
energy output of this magnitude could not be explained with
existing models, leading some researchers to talk about a GRB
energy crisis.
3) It turns out that the reality is more mundane. In November
2001, three groups reported at a workshop at Woods Hole (1) that
the initial energy estimates for GRBs were too high. The actual
GRB energy is narrowly distributed around a "mere" 10^(51) ergs.
The secret lies in beaming: The earlier energy estimates assumed
isotropic emission, but GRBs form beams, some with an opening
angle of only a few degrees. The wide distribution of observed
fluxes and apparent luminosities results mostly from variations
in these beaming angles.
4) According to the common fireball model (2,3), a GRB begins
when a compact "central engine" accelerates relativistic flow to
a velocity close to the speed of light. The kinetic energy of
this flow is dissipated by shocks within the flow, producing the
observed gamma rays. But these internal shocks do not dissipate
all the available energy. External collisions with surrounding
matter (interstellar matter or material ejected earlier from the
progenitor) slow down the flow that still carries away a large
fraction of the initial kinetic energy. The resulting external
shocks produce the afterglow.(4,5)
References (abridged):
1. Gamma-Ray Burst and Afterglow Astronomy 2001: A Workshop
Celebrating the First Year of the HETE Mission, Woods Hole, MA, 5
to 9 November 2001, organized by G. Ricker, Massachusetts
Institute of Technology. For further information, see
http://space.mit.edu/HETE/WH2001/web/
2. T. Piran, Phys. Rep. 314, 575 (1999)
3. P. Mészáros, Science 291, 79 (2001)
4. V. Connaughton, Astrophys. J., in press; preprint available at
http://xxx.lanl.gov/abs/astro-ph/0111564
5. F. A. Harrison et al., Astrophys. J. Lett. 523, L121 (1999)
Science 2002 295:986
Related Background:
EARLY OPTICAL EMISSION FROM THE GAMMA-RAY BURST OF 4 OCTOBER 2002
The following points are made by D.W. Fox et al (California
Institute of Technology, US):
1) Observations of the long-lived emission -- or "afterglow" --
of long-duration gamma-ray bursts place them at cosmological
distances, but the origin of these energetic explosions remains a
mystery. Observations of optical emission contemporaneous with
the burst of gamma-rays should provide insight into the details
of the explosion, as well as into the structure of the
surrounding environment. One bright optical flash was detected
during a burst(1), but other efforts(2,3) have produced negative
results.
2) The HETE-II spacecraft gamma-ray burst (GRB) mission triggered
on 4 October 2002 at 12:06:13.6 UT (4), and within minutes the
authors began observations of the 30-arcmin radius error circle
with the Palomar 48-inch Oschin Telescope (P48) and the Near
Earth Asteroid Tracking (NEAT) camera. When the authors compared
the first image, obtained 537 s after the burst, with subsequent
images at 721 s and 1,028 s post-burst and with Palomar Digital
Sky Survey (DPOSS) images, a new fading source was revealed about
100 times brighter than the faintest sources in DPOSS. The
authors promptly disseminated5 its position and magnitude via the
GRB Coordinates Network (GCN; see http://gcn.gsfc.nasa.gov ),
proposing it as the probable GRB counterpart.
3) In summary: The authors report the discovery of the optical
counterpart of GRB021004 only 193 seconds after the event. The
initial decline is unexpectedly slow and requires varying energy
content in the gamma-ray burst blastwave over the course of the
first hour. Further analysis of the X-ray and optical afterglow
suggests additional energy variations over the first few days.(5)
References (abridged):
1. Akerlof, C. et al. Observation of contemporaneous optical
radiation from a gamma-ray burst. Nature 398, 400-402 (1999)
2. Williams, G. G. et al. LOTIS search for early-time optical
afterglows: GRB 971227. Astrophys. J. 519, L25-L29 (1999)
3. Kehoe, R. et al. A search for early optical emission from
short- and long-duration gamma-ray bursts. Astrophys. J. 554,
L159-L162 (2001)
4. Shirasaki, Y. et al. GRB021004(= H2380): a long GRB localized
by HETE in near-real time. GRB Circ. Netw. 1565, 1 (2002)
5. Fox, D. W. GRB021004: optical afterglow. GRB Circ. Netw. 1564,
1 (2002)
Nature 2003 422:284
ON DARK GAMMA RAY BURSTS.
Gerald J. Fishman (NASA Marshall Space Flight Center, US)
discusses gamma ray bursts, the author making the following
points:
1) Gamma-ray bursts (GRBs) are intense flashes of radiation that
originate in the furthest reaches of the Universe. The bursts are
expected to be followed by an afterglow of radiation at optical
wavelengths. But for many GRBs (approximately 60% of the total
sample of less than 40 bursts that have now been followed up),
this afterglow seems to be missing. Hence, they have become known
as "dark" bursts. Berger et al(1) have reported data from an
armada of telescopes that reveal a faint optical afterglow from a
GRB that would otherwise have been classed as "dark", and their
observations suggest that the seeming lack of optical emission
from dark bursts is in fact a result of insufficiently prompt and
sensitive observations.
2) Astronomers once thought that GRBs occurred within our Galaxy,
but in the early 1990s it was realized that these explosive
events actually occur at cosmological (extra-galactic) distances,
of the order of 15 billion light years away. The distance issue
was settled by an observational breakthrough -- the accurate and
rapid location of GRBs by the Italian Dutch spacecraft Beppo SAX.
Following the initial detection and accurate location of gamma-
rays by Beppo SAX, the optical-radiation counterpart could be
determined by other telescopes. This also makes it possible to
measure the distance of the burst from the Earth. More
importantly, detailed follow-up observations could then be made
of the burst afterglow and of its host galaxy at other
wavelengths by some of the most powerful telescopes in the world,
both ground-based and space-based (in particular, the Hubble
Space Telescope and the Chandra X-ray Observatory).
3) The field of prompt GRB follow-up observations has burgeoned
in the past four years, in both the observational and the
theoretical areas. Afterglow observations are being made at X-
ray, optical, microwave, infrared and radio wavelengths. In each
observational band, and by combining data from different bands,
astronomers can make clever use of the data, such as providing a
measure of the angular size of the emitting region by observing
radio variability(2-5).
References (abridged):
1. Berger, E. et al. Astrophys. J. (in the press); Preprint
astro-ph/ 0207320, http://arXiv.org
2. Waxman, E., Kulkarni, S. & Frail, D. Astrophys. J. 497, 288-
293 (1998).
3. Hurley, K. et al. Astrophys. J. Suppl. 122, 497-501 (1999).
4. http://www.swift.psu.edu
5. http://www-glast.sonoma.edu
Nature 2002 419:259
Related Background:
A REASSESSMENT OF GAMMA RAY BURSTS
Gamma ray bursts are intense flashes of gamma rays detected at
energies up to 10^(6) *electronvolts. They were discovered by US
Air Force satellites in 1967 but not declassified until 1973. The
detection of these bursts averages about 1 per day, and
measurements indicate the distribution of bursts is isotropic,
i.e., they are uniformly distributed across the sky. Recent views
are that gamma ray bursts are produced by the merger of two
neutron stars, or a neutron star with a black hole, or a massive
star with a black hole. Up to this point, the bursts that have
been noted apparently originate outside our own galaxy.
Gamma rays are radiation of high energy, from about 10^(5)
*electronvolts to more than 10^(14) electronvolts -- radiation
with the shortest wavelengths and highest frequencies, the gamma
ray region of the electromagnetic spectrum merging into the
adjacent lower energy x-ray region.
Stan Woosley (University of California Santa Cruz, US) discusses
gamma ray bursts, the author making the following points:
1) Though sometimes touted as the largest explosions since the
Big Bang, gamma ray bursts are now seeming less energetic than
they did just two years ago, and recent observations suggest that
the energy required to produce these bursts may not be much more
than that of an ordinary supernova in which emitted energy is
somehow focused into a narrow jet moving very close to the speed
of light and pointing directly towards Earth. New evidence, based
on measurements of the size of the jets, suggests that all gamma
ray bursts release approximately the same amount of energy.
2) The energies inferred from gamma ray burst observations are
enormous, amounting in some cases to the mass of the Sun being
converted directly into gamma rays within a few seconds. In the
late 1990s, model builders struggled to reproduce these sorts of
burst energies in computer simulations, and wondered whether the
energies were really that large. In particular, would a gamma ray
burst be just as bright if it were seen from some other angle?
3) In the past decade, theoretical astrophysicists began
suggesting that much less energy would be needed to produce the
same brightness if gamma ray bursts broadcast their energy in a
narrow cone or jet. Most models of gamma ray burst formation
involve either the cataclysmic collapse of a massive rotating
star into a black hole, or a neutron star merging with a black
hole. Because the material accreting into the black hole forms a
rotating disk, it blocks the outflow of mass, so any material
that does escape [before being swallowed up by the black hole] is
forced into two narrow and oppositely directed jets. Other
astronomical phenomena thought to be powered by accreting black
holes, such as quasars, are also known to produce jets, so why
not gamma ray bursts? Recent experimental estimates of the
angular size of gamma ray bursts, based on observations of their
afterglows, suggests that even if we assume that the original
emission is spherical, matter released by a gamma ray burst is
accelerated close to the speed of light and so emits radiation in
a narrow beam along its path.
Nature 2001 414:853
Related Background:
GAMMA RAY BURSTS: THE LARGEST EXPLOSIONS IN THE UNIVERSE
Gamma rays are extremely high energy electromagnetic radiation
with wavelengths of less than approximately 0.01 nanometers. X-
rays are radiation of wavelengths approximately 0.01 to 10
nanometers, shorter than ultraviolet radiation but longer than
gamma rays. Gamma ray bursts are intense flashes of gamma rays
and x-rays detected at energies up to 10^(6) electron volts. They
were discovered by US Air Force satellites in 1967 but not
declassified until 1973. The detection of these bursts averages
approximately 1 per day, and measurements indicate the
distribution of bursts is isotropic, i.e., they are uniformly
distributed across the sky. The nature of gamma ray bursts
remains mysterious. Astronomers have obtained rigorous distance
estimates only recently, placing gamma ray bursts definitely in
the realm of cosmology. *Redshift measurements suggest extremely
large distances, making gamma ray bursts the most powerful
catastrophic energy releases known to mankind.
Related Background:
GAMMA-RAY BURSTS: THE LARGEST EXPLOSIONS IN THE UNIVERSE
Dieter H. Hartmann (Clemson University, US) presents a review of
current research concerning gamma ray bursts, the author making
the following points:
1) Gamma ray bursts are short flashes of almost pure high-energy
emission (x-rays and gamma rays) that occur randomly on the sky,
and from loci which apparently do not emit more than once.
Typical durations are of the order of seconds, but can range from
a few milliseconds to over 1000 seconds. The bursts are extremely
bright, outshining all other objects on the gamma ray sky, but
their spectra are featureless and reveal little about the
underlying physical processes. Integrating burst spectra over
energy and time yields large fluences (received energy per unit
area), but does not determine the total burst energy until the
distance is known.
2) Although the statistical properties of gamma ray bursts long
supported the idea that bursts occur at cosmological distances,
this distance scale was finally established by a burst on 8 May
1997, for which a faint extended object was optically identified
as the host, the object showing clear evidence of absorption
lines that indicated a lower redshift limit of z = 0.835. On 14
December 1997, another burst showed absorption lines at z = 3.42,
and then a third burst on 3 July 1998 had associated absorption
lines at z = 0.966 -- all of this indicating that gamma ray
bursts, along with quasars, are the most distant objects in the
Universe. Such large distances imply large energies, and in fact
the assumption of isotropic emission implies burst energies in
excess of 10^(53) ergs, comparable to supernova energies but
released predominantly in the gamma ray band. The optical
afterglows of gamma ray bursts are much brighter than supernova,
hence the name "hypernova" has been proposed.
3) Studies of gamma ray burst host galaxies suggest they are
normal star-forming galaxies, and not galaxies with active
nuclei. The estimated star formation rates in these hosts,
together with other evidence from x-ray spectra and photometry of
the gamma ray burst afterglows suggests that gamma ray bursts may
be directly associated with star-forming regions. If that turns
out to be correct, astronomers would have a powerful new tool for
the study of structure formation in the Universe, a tool that
could reach further back in time than quasars.
4) Despite recent breakthroughs in gamma ray burst observations,
many questions remain about the nature of the underlying
processes and the evolutionary sequences leading up to the
creation of the central engine driving these outbursts. The
ultimate goal of understanding this engine may be accomplished
through simultaneous optical observations, and such is the
objective of dedicated experiments under development throughout
the world.
Proc. Nat. Acad. Sci. 1999 96:4752
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6. RELATIVISTIC JETS
MAGNETOHYDRODYNAMIC PRODUCTION OF RELATIVISTIC JETS
The following points are made by D.L. Meier et al (California
Institute of Technology, US):
1) In this context, a "jet" is a tightly collimated stream of
fluid, gas, or plasma. It typically carries kinetic and internal
energy and linear momentum, and if it is set spinning about its
direction of motion by some means, it can carry angular momentum
as well. A relativistic jet is one whose speed approaches the
universally constant speed of light c = 299,792.5 km/s. At such
velocities, Einstein's theory of relativity becomes important.
The kinetic energy of motion (and possibly the internal thermal
and magnetic energy as well) adds mass to the jet, equal to
E(kinetic)/c^(2), making it more difficult to accelerate. Also,
as seen by viewers at rest, time slows down in the moving jet
material, and any light or radio emission from the jet tends to
be radiated in the direction of flow, not isotropically, as would
be the case if the flow velocity were subrelativistic.
2) In summary: A number of astronomical systems have been
discovered that generate collimated flows of plasma with
velocities close to the speed of light. In all cases, the central
object is probably a neutron star or black hole and is either
accreting material from other stars or is in the initial violent
stages of formation. Supercomputer simulations of the production
of relativistic jets have been based on a magnetohydrodynamic
model, in which differential rotation in the system creates a
magnetic coil that simultaneously expels and pinches some of the
infalling material. The model may explain the basic features of
observed jets, including their speed and amount of collimation,
and some of the details in the behavior and statistics of
different jet-producing sources.(1-5)
References (abridged):
1. R. C. Vermeulen, IAU Symp. 175, 57 (1996)
2. J. A. Biretta, W. B. Sparks, F. Macchetto, Astrophys. J. 520,
621 (1999)
3. F. Macchetto, et al., Astrophys. J. 489, 579 (1997)
4. I. F. Mirabel and L. F. Rodriguez, Annu. Rev. Astron.
Astrophys. 37, 409 (1999)
5.B. Margon, Annu. Rev. Astron. Astrophys. 22, 507 (1984)
Science 2001 291:84
Related Background:
EXTRACTION OF BLACK HOLE ROTATIONAL ENERGY BY A MAGNETIC FIELD
AND THE FORMATION OF RELATIVISTIC JETS
The following points are made by S. Koide et al (Toyama
University, JP):
1) Relativistic jets have now been discovered in several
different classes of astrophysical objects, including active
galactic nuclei (1,2), microquasars (3,4), and gamma ray bursts
(5). A rapidly spinning black hole may exist at the center of
each of these objects, and energetic reactions that occur near
the hole may be responsible for the jets. One of the most
promising processes for producing relativistic jets is the
extraction of rotational energy from a spinning (Kerr) black
hole. One method of extraction is the Penrose process, which uses
fission of a particle near the black hole to extract the black
hole rotational energy. However, this process may not be
applicable to most astrophysical objects, because the particle
fission must occur near the black hole, and the relative velocity
of the particles produced by the fission should be near the speed
of light.
2) On the other hand, Blandford and Znajek (1977) showed that a
large-scale magnetic field around a Kerr black hole also could
extract rotational energy. They assumed a magnetic force-free
condition, which corresponds to an extremely strong magnetic
field or an extremely low inertia plasma case. Recently, evidence
of the extraction of rotational energy from a Kerr black hole by
a magnetic field was suggested by observations of a broad Fe K
line in the bright Seyfert 1 galaxy MCG-6-30-15. Modeling of this
emission indicates that it is concentrated in a small central
disk region near the black hole. It is plausibly explained by a
model in which the black hole rotational energy is being
extracted into the disk by a magnetic field with a strength of
~10^(4) Gauss that connects the black hole to the disk.
3) To understand the basic physics of rotational energy
extraction from a black hole with a finite magnetic field, the
authors have investigated a somewhat simpler system using general
relativistic magnetohydrodynamic (MHD) numerical calculations.
Initially, the system consists of a Kerr black hole with a
uniform magnetic field, uniform plasma, and no accretion disk.
The calculations are based on the general relativistic
formulation of the laws of conservation of particle number and
energy momentum, Maxwell equations, and Ohm's law with zero
electrical resistance.
4) In summary: Using numerical simulations, the authors modeled
the general relativistic magnetohydrodynamic behavior of a plasma
flowing into a rapidly rotating black hole in a large-scale
magnetic field. The results show that a torsional Alfvén wave is
generated by the rotational dragging of space near the black
hole. The wave transports energy along the magnetic field lines
outward, causing the total energy of the plasma near the hole to
decrease to negative values. When this negative energy plasma
enters the horizon, the rotational energy of the black hole
decreases. Through this process, the energy of the spinning black
hole is extracted magnetically.
References (abridged):
1. T. J. Pearson, et al., Nature 290, 365 (1981)
2. J. A. Biretta, W. B. Sparks, F. Macchetto, Astrophys. J. 520,
621 (1999)
3. I. F. Mirabel and L. F. Rodriguez, Nature 371, 46 (1994)
.
4. S. J. Tingay, et al., Nature 374, 141 (1995)
5. S. R. Kulkarni, et al., Nature 398, 389 (1999)
Science 2002 295:1688
Notes:
A "Kerr black hole" has angular momentum but no charge (i.e., a
rotating black hole with no charge).
In general, an "Alfvén wave" is a disturbance transmitted through
a plasma (a fully ionized gas) in the presence of a magnetic
field. The direction of propagation is parallel to the mean
magnetic field, with the plasma particles vibrating at right
angles to this direction. The speed of propagation, the "Alfvén
speed", depends on the magnetic field strength and plasma
density. Such waves are a type of magnetohydrodynamic wave, and
they have been directly observed in solar wind high-speed streams
from the Sun and in planetary magnetospheres.
Related Background:
OBSERVATIONAL EVIDENCE FOR THE ACCRETION-DISK ORIGIN FOR A RADIO
JET IN AN ACTIVE GALAXY
The following points are made by Alan P. Marscher et al (Boston
University, US):
1) Accretion of gas onto black holes is thought to power the
relativistic jets of material ejected from active galactic nuclei
(AGN) and the "microquasars" located in our Galaxy(1-3). In
microquasars, superluminal radio-emitting features appear and
propagate along the jet shortly after sudden decreases in the X-
ray fluxes(1). This establishes a direct observational link
between the black hole and the jet: the X-ray dip is probably
caused by the disappearance of a section of the inner accretion
disk(4) as it falls past the event horizon, while the remainder
of the disk section is ejected into the jet, creating the
appearance of a superluminal bright spot(5). No such connection
has hitherto been established for AGN, because of insufficient
multi-frequency data.
2) The authors report the results of three years of monitoring
the X-ray and radio emission of the galaxy 3C120. As has been
observed for microquasars, the authors find that dips in the X-
ray emission are followed by ejections of bright superluminal
knots in the radio jet. The mean time between X-ray dips appears
to scale roughly with the mass of the black hole, although there
are at present only a few data points.
References (abridged):
1. Mirabel, I. F. & RodrÝguez, L. F. Microquasars in our Galaxy.
Nature 392, 673-676 (1998)
2. Greiner, J., Cuby, J. G. & McCaughrean, M. J. An unusually
massive stellar black hole in the Galaxy. Nature 414, 522-525
(2001)
3. Meier, D. L., Koide, S. & Uchida, Y. Magnetohydrodynamic
production of relativistic jets. Science 291, 84-92 (2000)
4. Belloni, T. Inner disk oscillations. Astrophys. Space Sci. 276
(suppl.), 145-152 (2001)
5. Gľmez, J. L., MartÝ, J. M., Marscher, A. P., Ibß±ez, J. M. &
Alberdi, A. Hydrodynamical models of superluminal sources.
Astrophys. J. 482, L33-L36 (1997)
Nature 2002 417:625
PLASMA JETS IN ACTIVE GALACTIC NUCLEI
A.P. Lobanov and J.A. Zensus (Max Planck Institute for Radio
Astronomy Bonn, DE) discuss plasma jets in active galactic
nuclei. One of the most intriguing features observed in active
galactic nuclei is highly collimated and relativistic plasma
outflows (jets) that originate in the immediate vicinity of the
center of activity and propagate at distances of up to several
megaparsecs (1 parsec = 3.26 light years). Observations of jets
in active galactic nuclei probe the behavior of extremely
relativistic matter in the Universe and provide a unique and
remote "laboratory" for studying the most powerful cosmic
phenomena such as supermassive black holes and extragalactic
accretion disks. The quasar 3C273 is one of the closest and most
luminous and best studied active galactic nuclei, with a
prominent relativistic outflow observed in the x-ray, optical,
and radio wave bands. The relativistic jet observed in this
quasar is one-sided, with no signs of emission on the counterjet
side at dynamic ranges of up to 16,000:1. This is evidence for
strong relativistic boosting in an intrinsically double-sided
outflow powered by an accretion disk around a black hole. The
enhanced emission features (jet components) identified in the jet
on scales of up to approximately 20 milli-arc seconds are moving
at apparent speeds exceeding the speed of light by factors of 5
to 8. These jet components may result from the flares observed in
this quasar in the optical and radio wavelengths and also reflect
the precession of the jet axis. The structure and kinematics of
such outflows are typically explained in terms of shock waves and
Kelvin-Helmholtz instability.
Science 2001 294:128
Related Background:
ASTROPHYSICS: ON RELATIVISTIC JETS
The term "Seyfert galaxy", named after Carl K. Seyfert (1911-
1960) refers to a type of galaxy with a small bright nucleus
exhibiting broad strong emission lines in its spectrum. Seyfert
galaxies apparently have active galactic nuclei that produce the
strong radiation, and these galaxies may be examples of low-
luminosity quasar activity.
The unmanned satellite called the "Chandra X-Ray Observatory" was
launched in 1999. This instrument was formerly called the
Advanced X-ray Astrophysics Facility, but it was renamed in honor
of the astrophysicist Subrahmanyan Chandrasekhar (1910-1995). The
Chandra instrument is equipped with a nested array of mirrors to
focus x-rays on two cameras that can produce highly detailed
images or high-resolution spectra of sources of x-ray emissions.
A.C. Fabian (University of Cambridge, UK) discusses relativistic
jets, the author making the following points:
1) Most galaxies, including our own, appear to have a massive
black hole at their center. Some, the active galactic nuclei, are
visible to us as a result of luminous outpourings such as quasars
and the less powerful Seyfert galaxies. Approximately 10% of
active galactic nuclei are strong radio emitters. Diametrically
opposed pairs of powerful energetic jets squirt out of these
"radio-loud" objects at relativistic speeds. The jets themselves
are rarely detected: the strongest evidence for their existence
may be a pair of lobes of radio-emitting material on either side
of the nucleus where the jets are shocked and decelerated by
surrounding gas.
2) The first of these jets was sighted by H. Curtis almost 90
years ago in the giant elliptical galaxy M87 in the Virgo
cluster. Twin radio lobes were reported almost 50 years ago. Yet
the matter content of jets (apart from the emitting electrons),
their total power, and their evolution remain uncertain. The high
spatial resolution x-ray imaging achieved by NASA's Chandra
observatory is, however, beginning to answer these questions. X-
ray observations have proven so useful because the gaseous
atmospheres that fuel the jets usually have temperatures on the
order of 10^(6) to 10^(7) kelvins. By studying the surrounding
gas, particularly bubbles in the gas blown by the jets, their
total power and history over the past 10^(7) to 10^(8) years can
be deduced. Holes and depressions in the x-ray emission have now
been seen around radio sources in numerous clusters, including
the Virgo cluster, the Perseus cluster, Hydra A, and the
Centaurus cluster (1-5).
3) The relativistic jets are believed to originate very close to
the central black hole and are probably ejected up the rotation
axis of the accreting gas, or the spin axis of the black hole
itself. As the jet decelerates in the surrounding matter, a
bubble of low-density heated gas and relativistic plasma
accumulates about the end of the jet. Unless the jet is so
powerful that it dominates the hot gas, the bubble expands until
buoyancy forces cause it to rise up and break away as a new
bubble forms. The situation is similar to that of a dripping tap,
with relative densities and directions reversed.
References (abridged):
1. H. Bohringer et al., Mon. Not. R. Astron. Soc. 264, L25 (1993)
2. A. C. Fabian et al., Mon. Not. R. Astron. Soc. 318, L65 (2000)
3. B. R. McNamara et al., Astrophys. J. 534, L135 (2000)
4. J. S. Sanders, A. C. Fabian, Mon. Not. R. Astron. Soc. 331,
273 (2002)
5. A. J. Young, A. S. Wilson, C. G. Mundell, in preparation
(available at http://xxx.lanl.gov/abs/astro-ph/0202504)
Science 2002 296:1040
Related Background:
ON ASTROPHYSICAL JETS
In this context, the term "poloidal magnetic field" refers to a
magnetic field generated by an electric current flowing in a
ring. (A "toroidal magnetic field" is an electric field generated
by a current flowing in a solenoid round a torus.)
Mario Livio (Space Telescope Science Institute, US) discusses
astrophysical jets, the author making the following points:
1) Although a wide variety of astrophysical objects produce
powerful jets, we still lack a comprehensive theory of their
formation. In our Galaxy, young stellar objects, massive X-ray
binaries, black hole X-ray transients, symbiotic stars, supersoft
X-ray sources and even some planetary nebulae are all accreting
systems that produce jets. Among extragalactic sources, powerful
jets are observed in active galactic nuclei (accreting,
supermassive black holes) and are thought to exist in -ray bursts
(which possibly involve accreting, stellar-mass black holes).
2) Despite the ubiquity of jets, there is no generally accepted
theory for the mechanism of their acceleration and alignment.
Early attempts to explain this collimation involved nozzles
(similar to rocket exhausts), in which an adiabatic flow
propagating in a medium with decreasing pressure first converges
and then diverges as it accelerates to supersonic speed. Another
idea was that the collimating agents are funnels that are formed
at the centres of dense tori. Radiation pressure on
electron–positron pairs, on resonance lines, or on dust, have
been suggested to accelerate jets in some of the systems
mentioned above.
3) The main problem of these early models is that they do not
work for all classes of astrophysical jets. For example, although
the power emitted by the central source approaches the critical
Eddington luminosity (at which radiation pressure exactly
balances gravity) in supersoft X-ray sources, this is not true
for most other classes of objects. The drag caused by radiation
limits the attainable speeds to values below those observed in
other (ultrarelativistic) jets. Similarly, dense tori with narrow
axisymmetric funnels, which were once assumed to exist around
black holes, were later shown to be unstable to non-axisymmetric
instabilities that could destroy them on a dynamic timescale. An
important question, therefore, is whether a universal mechanism
of acceleration and collimation that operates in all classes can
be found? At the present time, the most promising universal
mechanism for jet acceleration and collimation relies on an
accretion disc threaded by a poloidal, large-scale magnetic
field.(1-4)
References (abridged):
1. Mirabel, I. F. & Rodriguez, L. F. Nature 392, 673–676 (1998).
2. Wilms, J. et al. Mon. Notices R. Astron. Soc. 328, L27–L31
(2001).
3. Celotti, A. & Blandford, R. D. in Black Holes in Binaries and
Galactic Nuclei: Diagnostics, Demography and Formation (eds
Kaper, L. et al.) 206–215 Springer, Berlin, 2001).
4. Livio, M. in Probing the Physics of Active Galactic Nuclei by
Multiwavelength Monitoring (eds Peterson, B. M. et al.) 225–248
(ASP, San Francisco, 2001).
Nature 2002 417:125
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