ScienceWeek

ASTROPHYSICS: ON ULTRA-RELATIVISTIC OUTFLOWS

The following points are made by Rob Fender et al (Nature 2004 427:222):

1) Collimated relativistic outflows -- also known as "jets" --are amongst the most energetic phenomena in the Universe. They are associated with supermassive black holes in distant active galactic nuclei(1), accreting stellar-mass black holes, and neutron stars in binary systems(2), and they are also believed to be responsible for gamma-ray bursts(3). The physics of these jets, however, remains something of a mystery in that their bulk velocities, compositions and energetics remain poorly determined.

2) Circinus X-1 is a bright and highly variable X-ray source, containing a stellar-mass compact object accreting from a binary companion star. X-ray dips and outbursts with a period of 16.6 days are most readily interpreted as enhanced accretion during periastron passage of the accreting object in a significantly elliptical binary orbit(4,5). Observations of type I X-ray bursts indicate that the accreting object is a neutron star. In the 1970s Cir X-1 was also associated with bright radio outbursts. Since then, there has been a decline in the strength of the radio counterpart, which has been found to reside within an extended radio nebula. Structures on arcminute scales have been imaged within this nebula, and more recently a one-sided jet on arcsecond scales, which aligns with the larger structures, has been discovered.

3) In summary: The authors report the discovery of an ultra-relativistic outflow from a neutron star (Circinus X-1) accreting gas within a binary stellar system. The velocity of the outflow is comparable to the fastest-moving flows observed from active galactic nuclei, and its strength is modulated by the rate of accretion of material onto the neutron star. Shocks are energized further downstream in the flow, which are themselves moving at mildly relativistic bulk velocities and are the sites of the observed synchrotron emission from the jet. The authors conclude that the generation of highly relativistic outflows does not require properties that are unique to black holes, such as an event horizon.

References (abridged):

1. Ostrowski, M., Sikora, M., Madejski, G. & Begelman, M. (eds) Relativistic Jets in AGNs (Towarzystwa Salezjaskiego, Krakow, 1997)

2. Mirabel, I. F. & Rodriguez, L. F. Sources of relativistic jets in the Galaxy. Annu. Rev. Astron. Astrophys. 37, 409-443 (1999)

3. Sari, R., Piran, T. & Halpern, J. P. Jets in gamma-ray bursts. Astrophys. J. 519, L17-L20 (1999)

4. Kaluzienski, L. J., Holt, S. S., Voldt, E. A. & Serlemitsos, P. J. Evidence for a 16.6 day period from Circinus X-1. Astrophys. J. 208, L71-L75 (1976)

5. Johnston, H. M., Fender, R. & Wu, K. High-resolution optical and infrared spectroscopic observations of CIR X-1. Mon. Not. R. Astron. Soc. 308, 415-423 (1999)

Nature http://www.nature.com/nature

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In this context, a "jet" is a long thin linear feature of bright emission extending from a compact object. Jets are very common at radio wavelengths, but have also been observed in optical and x-ray emissions. A "relativistic jet" is a jet moving at close to the speed of light.

A "Kerr black hole" has angular momentum but no charge (i.e., a rotating black hole with no charge).

In general, an "Alfven 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 "Alfven 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.

The boundary of a black hole is called the "event horizon" (black hole horizon), because any event within the boundary is invisible outside, the invisibility resulting from the fact that no radiation can escape to be detected.

Some galaxies are known to have very "active" central regions from which enormous amounts of energy are emitted each second, and it is believed that these "active galactic nuclei" are probably powered by accretion of matter into a supermassive black hole of 10^(6) to 10^(9) solar-masses. Astronomers have recently discovered that many active galactic nuclei eject clouds of ionized gas with velocities of up to 10 percent of the speed of light over a wide range of angles, in contrast to the previously known collimated jets. These mass outflows are considered to be intriguing because they provide information about the dynamical forces (such as radiation and wind pressure) near an active supermassive black hole.

Quasars (quasi-stellar objects) 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. They are believed to involve massive black holes. Microquasars are quasars of apparent stellar mass.

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. The current consensus is that gamma ray bursts are produced by the merger of two neutron stars, and up to this point, the bursts that have been noted apparently originate outside our own galaxy.

MAGNETOHYDRODYNAMIC PRODUCTION OF RELATIVISTIC JETS.

The following points are made by D.L. Meier et al (Science 2001 291:84):

1) 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 per second. 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. Because c is a maximum speed limit and because conditions become more extreme as it is approached, the Lorentz factor is often used to characterize the speed, rather than the velocity.

2) Relativistic jets are common in the astrophysical environment. The extragalactic radio sources such as radio galaxies and quasars produce by far the largest and most energetic jets in the universe, although they do not produce the fastest ones nor those with the highest instantaneous powers.(1) The less luminous ones appear as giant elliptical radio galaxies, and the most luminous appear as radio quasars. Often, the twin jets are pointed at a large angle to our line of sight, allowing the full extent of the radio source powered by the jet -- up to a few megaparsecs in size -- to be seen. In a few sources, however, one of the jets points nearly directly toward Earth and the other points nearly directly away. The approaching jet therefore appears to be substantially Doppler brightened, and the receding one may be so Doppler dimmed that it is difficult or impossible to detect.

3) 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.(2-5)

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 http://www.sciencemag.org

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SOURCES OF RELATIVISTIC JETS IN THE GALAXY.

The following points are made by I. F. Mirabel and L. F. Rodriguez (Annu. Rev. Astron. Astrophys. 1999 37:409):

1) While the first evidence of jet-like features emanating from the nuclei of galaxies goes back to the discovery by Curtis (1918) of the optical jet from the elliptical galaxy M87 in the Virgo cluster, the finding that jets can also be produced in smaller scale by binary stellar systems is much more recent. The detection by Margon et al (1979) of large, periodic Doppler drifts in the optical lines of SS 433 resulted in the proposition of a kinematic model (Fabian & Rees 1979; Milgrom 1979) consisting of two precessing jets of collimated matter with velocity of 0.26c. High angular radio imaging as a function of time showed the presence of outflowing radio jets and fully confirmed the kinematic model (Spencer 1979; Gilmore & Seaquist 1980; Gilmore et al 1981; Hjellming & Johnston 1981). The early history of SS 433 has been reviewed by Margon (1984).

2) Since the detection of Sco X-1 at radio wavelengths (Ables 1969), some X-ray binaries had been known to be strong, time-variable non-thermal emitters. Ejection of synchrotron-emitting clouds was suspected from those days, but the actual confirmation of radio jets came only with the observations of SS 433. At present, there are about 200 known galactic X-ray binaries (van Paradijs 1995), of which about 10 percent are radio-loud (Hjellming & Han 1995). Of these radio-emitting X ray binaries, 10 have shown evidence of relativistic jets of synchrotron emission.

3) In the last years it has become clear that collimated ejecta can be produced in several stellar environments when an accretion disk is present. Jets with terminal velocities in the order of a few hundred to a few thousand km per sec are now known to emanate from objects as diverse as very young stars (Reipurth & Bertout 1997), nuclei of planetary nebulae (L½pez 1997), and accreting white dwarfs that appear as supersoft X-ray sources (Motch 1998, Cowley et al 1998). These types of stellar jets have, however, non-relativistic velocities (~100 10000 km per sec) and their associated emission is dominantly thermal (i.e. free-free continuum emission in the radio as well as characteristic near-IR, optical and UV lines). Interestingly, in all known types of jet sources a disk is believed to be present.

4) In summary: Black holes of stellar mass and neutron stars in binary systems are first detected as hard X-ray sources using high-energy space telescopes. Relativistic jets in some of these compact sources are found by means of multiwavelength observations with ground-based telescopes. The X-ray emission probes the inner accretion disk and immediate surroundings of the compact object, whereas the synchrotron emission from the jets is observed in the radio and infrared bands, and in the future could be detected at even shorter wavelengths. Black-hole X-ray binaries with relativistic jets mimic, on a much smaller scale, many of the phenomena seen in quasars and are thus called microquasars. Because of their proximity, their study opens the way for a better understanding of the relativistic jets seen elsewhere in the Universe. From the observation of two-sided moving jets it is inferred that the ejecta in microquasars move with relativistic speeds similar to those believed to be present in quasars. The simultaneous multiwavelength approach to microquasars reveals in short timescales the close connection between instabilities in the accretion disk seen in the X-rays, and the ejection of relativistic clouds of plasma observed as synchrotron emission at longer wavelengths. Besides contributing to a deeper understanding of accretion disks and jets, microquasars may serve in the future to determine the distances of jet sources using constraints from special relativity, and the spin of black holes using general relativity.

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