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ScienceWeek
ASTROPHYSICS: ON MICROBLAZARS
The following points are made by Wei Cui (Science 2005 309:714):
1) Active galactic nuclei (AGN) are some of the most powerful objects in the Universe, and they are almost certainly powered by very massive black holes at the center of galaxies. As clouds, stars, or other material fall into the black hole, some of their gravitational energy is converted into radiation that we detect on Earth. The accretion of matter by the black hole is sometimes accompanied by well-collimated and powerful outflows of material, also known as jets. Blazars are an important subset of AGN whose jets are closely aligned with our line of sight. New work[1] reports the possible detection of a long-sought downsized version of these objects called a "microblazar".
2) Such microblazars could hypothetically exist, given that there is growing observational evidence for the presence of micro-AGN [2] powered by black holes less than one-millionth the mass of those in blazars. These micro-AGN belong to a more general class of sources known as xray binaries, which are some of the brightest x-ray sources in the sky. An x-ray binary consists of a stellar-mass black hole or a neutron star and a normal star bound together by their mutual gravitational attraction. If a black hole system also produces jets, it is referred to as a microquasar, and many have been discovered over the past decade. A microquasar whose jet points at us would be a microblazar. Although circumstantial evidence exists, the presence of microblazars has not yet been firmly established observationally.
3) One way to find microblazars is perhaps to look for very energetic gamma rays from known (or candidate) microquasars. Some positive detections were claimed but were nearly always disputed by independent observations with a different (sometimes more sensitive) instrument or dismissed on other grounds. It is fair to say that there has not be any credible evidence for the detection of TeV gamma rays from any microquasar until now. Aharonian et al [1] present a careful analysis of the fields in their survey of the Milky Way with the High Energy Stereoscopic System (HESS) array that covered the position of a microblazar candidate, LS 5039. The image they show indicates a clear detection of the source at TeV energies. The question remains whether LS 5039 is truly a microblazar. The authors are cautious in this regard.
4) LS 5039 is considered a microblazar candidate because it has been identified as a counterpart to one of the still-mysterious sources detected at GeV energies by the Energetic Gamma Ray Experiment Telescope (EGRET) instrument aboard the Compton Gamma Ray Observatory. It is worth noting that most of the identified EGRET sources are in fact blazars. LS 5039 was subsequently observed and detected at x-ray and radio wavelengths. An important recent breakthrough came from direct imaging of the jets in the source at radio wavelengths [3], lending support to its microblazar candidacy. However, unlike typical blazars, the jets in LS 5039 appear to be only very mildly relativistic; the effects of relativistic beaming, which play an important role in blazars, should therefore not be relevant here. Nevertheless, the detection of radio jets in LS 5039 has generated much excitement and led to detailed studies of physical mechanisms to produce very energetic gamma rays in microblazars [4,5].
References (abridged):
1. F. Aharonian et al., Science 309, 746 (2005)
2. I. F. Mirabel, L. F. Rodriguez, Annu. Rev. Astron. Astrophys. 37, 409 (1999)
3. J. M. Paredes, J. Marti, M. Ribo, M. Massi, Science 288, 2340 (2000)
4. M. M. Kaufman Bernadó, G. E. Romero, I. F. Mirabel, Astron. Astrophys. 385, L10 (2002)
5. G. E. Romero, D. F. Torres, M. M. Kaufman Bernadó, I. F. Mirabel, Astron. Astrophys. 410, L1 (2003)
Science http://www.sciencemag.org
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ASTROPHYSICS: ON ACTIVE GALACTIC NUCLEI
The following points are made by Julian Krolik (Nature 2004 429:29):
1) Active galactic nuclei are among the most exotic objects in the Universe. They radiate as much light as an entire galaxy from a region the size of the Solar System and, unlike stars, spread that light over the entire electromagnetic spectrum, from radio waves to gamma-rays. Thanks to observational advances in the past decade, we now have good evidence that the "engine" powering each of these objects is a black hole, weighing anywhere from millions to billions of times as much as the Sun. Fortunately for the galaxies that house them (but unfortunately for distant observers like us), active galactic nuclei are often shrouded in opaque gas and dust that block a view of them from most directions. The light from them, intercepted by these dust clouds, is degraded to infrared light that tells us little about the fascinating activity deep inside.
2) To the further frustration of astronomers, the dust clouds are so close to the active nucleus that their structure cannot easily be made out: rough estimates put their typical size on the sky at 0.01 arcseconds (a few hundred-millionths of a degree), and even the Hubble Space Telescope cannot resolve anything smaller than about 0.1 arcseconds. But this barrier is at last being breached: Jaffe et al(1) have presented infrared images of an active nucleus that have a resolution of 0.01 arcseconds, achieved through interferometry -- the careful combination of images from different telescopes placed some distance apart. The obscuring dust clouds, a scant several light years from a supermassive black hole, have now come into view.
3) Light can be thought of either as a collection of individual energy packets called photons or as an electromagnetic wave. Interferometry exploits the wave aspect of light in that it hinges on comparing the phases of electromagnetic waves from a single source when they strike different telescopes. Measuring and retaining this phase information is a formidable technical challenge, so interferometry is not a discipline for the faint-hearted. Although the technique has been in regular use for decades at radio wavelengths, the difficulties increase as the wavelength of the light shrinks. The advance into the infrared region of the spectrum has been accomplished only recently(2) and, as is usually the case, the first observations were only of nearby, comparatively bright stars(3-5).
4) Jaffe et al(1) have used the interferometer formed by the European Southern Observatory's Very Large Telescope and several smaller telescopes on the same Chilean mountain. With this apparatus, high-resolution infrared images of faint, distant objects have been obtained for the first time. Indeed, the data shown in the Jaffe et al report(1) are the first to be derived from infrared interferometry on anything outside the Milky Way. The images show a geometrically thick ring of dust that is extremely close to the central black hole in the nearest powerful active galaxy, NGC 1068. The warmest dust is no more than two or three light years from the very centre of the beast.
5) This result is a dramatic confirmation of inferences made many years ago, but which were not entirely accepted because they were so hard to understand. Since the late 1980s, many have believed that the dust clouds surrounding active galactic nuclei are not amorphous, but are instead arranged more or less like a thick doughnut. As a result, observers whose sight-lines fall near to the equatorial plane of any particular active galactic nucleus (most of the Universe, in fact) are relegated to seats with an obscured view of its centre; the favored minority close to the axis of the doughnut get to see the full show unobstructed.
References (abridged):
1. Jaffe, W. et al. Nature 429, 47-49 (2004)
2. Townes, C. et al. Proc. SPIE 3350, 908-932 (1998)
3. Bester, M. et al. Astrophys. J. 463, 336-343 (1996)
4. Lopez, B. et al. Astrophys. J. 488, 807-826 (1997)
5. Malbet, F. et al. Astrophys. J. 507, L149-L152 (1998)
Nature http://www.nature.com/nature
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PLASMA JETS IN ACTIVE GALACTIC NUCLEI
Notes by ScienceWeek:
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.
The following points are made by A.P. Lobanov and J.A. Zensus (Science 2001 294:128):
1) 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.
2) 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.
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