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ScienceWeek
2004 11 June A1 ASTRONOMY: ON THE MILKY WAY CENTRAL BLACK HOLE
The following points are made by Mark R. Morris (Science 2004 304:689):
1) Precise observations in recent years at radio, infrared, and x-ray wavelengths have provided enough compelling evidence for most astronomers to subscribe to the awe-inspiring notion that a supermassive black hole resides at the center of our Milky Way Galaxy. With a mass 4 million times that of our Sun, it exerts a dramatic gravitational influence on the motions of objects passing within its several-light-year sphere of influence. As extensive as this domain may seem, it is quite modest from our distant perspective, 25,000 light-years away; the angular size of the black hole's domain is about as large, projected onto the sky, as the disk of nearby Jupiter, about 30 arc sec. The black hole itself is vastly smaller, so obtaining an unambiguous measurement of its extent has long been a major challenge.
2) It is traditional to specify the size of a black hole in terms of the size of its event horizon, the spacetime boundary defining the interior region from which nothing, including light, can escape. In these terms, the radius of the Galactic black hole is only 8% of the radius of Earth's orbit around the Sun (2). At the distance of the Galactic center, this corresponds to an angular size of only 10 millionths of an arc sec (10 microarcsec), which is currently beyond our capability to resolve. However, if we try to observe a black hole, then by definition, it is not the black hole itself that can be detected, but rather the luminous matter in its immediate environment.
3) Because of its unavoidable store of angular momentum, interstellar gas drawn toward the black hole naturally forms a rotating disk. Proximity to the black hole causes the innermost portions of such a disk to rotate at a higher angular rate than the outer portions, which leads to a viscous interaction between disk gas at different radii. This viscous drag causes some of the gas to lose angular momentum and spiral inwards, ultimately through the event horizon, to be added to the mass of the black hole. As gas is thus accreted onto the black hole, it is compressed and heated to extremely high temperatures, high enough that it emits radiation across the electromagnetic spectrum from radio to x-ray wavelengths.
4) Because the accretion disk is invariably magnetized and probably quite tumultuous -- characterized by strong shocks and various magnetohydrodynamic instabilities -- the actual emission processes can be quite complex, involving relativistic electrons that are emitting some combination of thermal bremsstrahlung emission, synchrotron emission, and Compton-scattered photons (3). To embellish the picture yet further, the accretion disk is likely to be driving a strong, collimated wind or even a high-speed jet of hot matter off its surface. So either or both the disk and the jet can be radiating at different wavelengths.
5) Consequently, when one peers at a black hole, it is the accretion disk, and perhaps the base of the jet, that one actually sees: typically, the region within a few dozen times the radius of the black hole event horizon. Bower et al (1) report a mapping of the radio emission immediately surrounding the black hole, which brings us as close as we have ever been to imaging the black hole itself.
References:
1. G. C. Bower et al., Science 304, 704 (2004)
2. F. Melia, The Black Hole at the Center of Our Galaxy (Princeton Univ. Press, Princeton NJ, 2003)
3. F. Melia, H. Falcke, Annu. Rev. Astron. Astrophys. 39, 309 (2001)
Science http://www.sciencemag.org
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A STAR IN A 15.2-YEAR ORBIT AROUND THE SUPERMASSIVE BLACK HOLE AT THE CENTER OF THE MILKY WAY
The following points are made by R. Schoedel et al (Nature 2002 419:694):
1) Many galaxies are thought to have supermassive black holes at their centers(1) -- more than a million times the mass of the Sun. Measurements of stellar velocities(2-5) and the discovery of variable X-ray emission have provided strong evidence in favor of such a black hole at the center of the Milky Way, but have hitherto been unable to rule out conclusively the presence of alternative concentrations of mass.
2) For the past ten years, the authors have been carrying out high-resolution near-infrared imaging and spectroscopy of the central few light years of our Milky Way for a detailed study of the stellar dynamics in the vicinity of the compact radio source SgrA* (2,3,5), the most likely counterpart of the putative black hole. From a statistical analysis of the stellar proper motions (velocities on the plane of the sky derived from multi-epoch imaging data) and line-of-sight velocities (Doppler motions derived from spectral lines) the authors deduced the presence of a mass of about 2.6 to 3.3 million solar masses (M) concentrated within ten light days of SgrA* (2,3,5). To further improve the sensitivity (by about 20) and the angular resolution/astrometric precision of the study (by about 3), the authors began this year to use the new COud‚ near-infrared camera (CONICA)/Nasmyth adaptive optics system (NAOS) imager/spectrometer on the 8-m UT4 (Yepun) of the European Southern Observatory (ESO) Very Large Telescope (VLT)11-13.
3) In summary: The authors report ten years of high-resolution astrometric imaging that allows them to trace two-thirds of the orbit of the star currently closest to the compact radio source (and massive black-hole candidate) Sagittarius A*. The observations, which include both pericenter and epicenter passages, show that the star is on a bound, highly elliptical Keplerian orbit around Sgr A*, with an orbital period of 15.2 years and a epicenter distance of only 17 light hours. The orbit with the best fit to the observations requires a central point mass of (3.7 +- 1.5) x 10^(6) solar masses (M). The authors suggest the data no longer allow for a central mass composed of a dense cluster of dark stellar objects or a ball of massive, degenerate fermions.
References (abridged):
1. Kormendy, J. & Richstone, D. Inward bound--The search for supermassive black holes in galactic nuclei. Annu. Rev. Astron. Astrophys. 33, 581-624 (1995)
2. Eckart, A. & Genzel, R. Observations of stellar proper motions near the Galactic Center. Nature 383, 415-417 (1996)
3. Genzel, T., Eckart, A., Ott, T. & Eisenhauer, F. On the nature of the dark mass in the center of the Milky Way. Mon. Not. R. Soc. 291, 219-234 (1997)
4. Ghez, A., Klein, B. L., Morris, M. & Becklin, E. E. High proper-motion stars in the vicinity of Sagittarius A*: Evidence for a supermassive black hole at the center of our galaxy. Astrophys. J. 509, 678-686 (1998)
5. Genzel, R., Pichon, C., Eckart, A., Gerhard, O. & Ott, T. Stellar dynamics in the Galactic Center: proper motions and anisotropy. Mon. Not. R. Soc. 317, 348-374 (2000)
Nature http://www.nature.com/nature
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RAPID X-RAY FLARING FROM THE DIRECTION OF THE SUPERMASSIVE BLACK HOLE AT THE GALACTIC CENTER
The following points are made by F.K. Baganoff et al (Nature 2001 413:45):
1) The nuclei of most galaxies are now believed to harbor supermassive black holes(1). The motions of stars in the central few light years of our Milky Way Galaxy indicate the presence of a dark object with a mass of about 2.6 x 10^(6) solar masses (2,3). This object is spatially coincident with the compact radio source Sagittarius A* (Sgr A*) at the dynamical center of the Galaxy, and the radio emission is thought to be powered by the gravitational potential energy released by matter as it accretes onto a supermassive black hole(4,5). Sgr A* is, however, much fainter than expected at all wavelengths, especially in X-rays, which has cast some doubt on this model. The first strong evidence for X-ray emission was found only recently.
2) Our view of Sgr A* in the optical and ultraviolet wavebands is blocked by the large visual extinction caused by dust and gas along the line of sight. Sgr A* has not been detected in the infrared owing to its faintness and to the bright infrared background from stars and clouds of dust. We thus need to detect X-rays from Sgr A* in order to constrain the spectrum at energies above the radio-to-submillimeter band and to test whether gas is accreting onto a supermassive black hole.
3) In summary: The authors report the discovery of rapid X-ray flaring from the direction of Sgr A*, which, together with the previously reported steady X-ray emission, provides compelling evidence that the emission is coming from the accretion of gas onto a supermassive black hole at the Galactic Center.
References (abridged):
1. Richstone, D. et al. Supermassive black holes and the evolution of galaxies. Nature 395 (suppl. on optical astronomy) A14-A19 (1998)
2. Genzel, R., Pichon, C., Eckart, A., Gerhard, O. E. & Ott, T. Stellar dynamics in the Galactic Center: proper motions and anisotropy. Mon. Not. R. Astron. Soc. 317, 348-374 (2000)
3. Ghez, A. M., Morris, M., Becklin, E. E., Tanner, A. & Kremenek, T. The accelerations of stars orbiting the Milky Way's central black hole. Nature 407, 349-351 (2000)
4. Lynden-Bell, D. & Rees, M. J. On quasars, dust and the Galactic center. Mon. Not. R. Astron. Soc. 152, 461-475 (1971)
5. Melia, F. & Falcke, H. The supermassive black hole at the Galactic Center. Annu. Rev. Astron. Astrophys. 39, 309-352 (2001)
Nature http://www.nature.com/nature
ScienceWeek http://scienceweek.com
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