* Astronomy

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RE: Sagittarius A*

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Black hole found at center of galaxy

German astronomers say they have discovered conclusive proof of a supermassive black hole at the heart of the galaxy. The study also enabled astronomers to calculate the distance of the earth from the center of the galaxy, now measured to be 27,000 light-years, and enhanced by six times the accuracy to which they were able to measure the positions of stars -- the equivalent of seeing a one euro coin from a distance of 10,000 kilometers. One star, called S2, orbited the center of the Milky Way so quickly that it completed one full revolution within the duration of the study.

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Title: Simultaneous NIR/sub-mm observation of flare emission from SgrA*
Authors: A. Eckart, R. Schoedel, M. Garcia-Marin, G. Witzel, A. Weiss, F. K. Baganoff, M. R. Morris, T. Bertram, M. Dovciak, W. J. Duschl, V. Karas, S. Koenig, T. P. Krichbaum, M. Krips, D. Kunneriath, R.S. Lu, S. Markoff, J. Mauerhan, L. Meyer, J. Moultaka, K. Muzic, F. Najarro, J. U. Pott, K. F. Schuster, L. O. Sjouwerman, C. Straubmeier, C. Thum, S. N. Vogel, H. Wiesemeyer, M. Zamaninasab, J. A. Zensus

We report on a successful, simultaneous observation and modelling of the sub-millimeter to near-infrared flare emission of the Sgr A* counterpart associated with the super-massive black hole at the Galactic centre. Our modelling is based on simultaneous observations that have been carried out on 03 June, 2008 using the NACO adaptive optics (AO) instrument at the ESO VLT and the LABOCA bolometer at the APEX telescope. Inspection and modelling of the light curves show that the sub-mm follows the NIR emission with a delay of 1.5 ±0.5 hours. We explain the flare emission delay by an adiabatic expansion of the source components.

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Astronomers detect matter torn apart by black hole
Astronomers have used two different telescopes simultaneously to study the violent flares from the supermassive black hole in the centre of the Milky Way. They have detected outbursts from this region, known as Sagittarius A*, which reveal material being stretched out as it orbits in the intense gravity close to the central black hole.
The team of European and US astronomers used ESO's Very Large Telescope (VLT) and the Atacama Pathfinder Experiment (APEX) telescope, both in Chile, to study light from Sagittarius A* at near-infrared wavelengths and the longer submillimetre wavelengths respectively. This is the first time that astronomers have caught a flare with these telescopes simultaneously. The telescopes' location in the southern hemisphere provides the best vantage point for studying the Galactic Centre.

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An international team, led by astronomers at the MIT Haystack Observatory, has obtained the closest views ever of what is believed to be a super-massive black hole at the center of the Milky Way galaxy.
The astronomers linked together radio dishes in Hawaii, Arizona and California to create a virtual telescope more than 2,800 miles across that is capable of seeing details more than 1,000 times finer than the Hubble Space Telescope.

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For a while now scientists have thought a dense, massive object lurking at the centre of our galaxy is likely a giant black hole, but they haven't been able to prove it. New observations, offering the closest view yet of the heart of the Milky Way, present strong evidence for the black hole theory and raise hopes of finally settling the question soon.
By linking a series of radio telescopes around the world, astronomers created a virtual telescope with the resolving power of a single dish the size of the distance between the various sites (about 2,800 miles, or 4,500 kilometres). This instrument grabbed an intimate image that probed nearly to the Milky Way's black hole's event horizon - the point beyond which nothing, including light, could escape.

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Title: On the Formation of Compact Stellar Disks Around Sgr A*
Authors: Mark Wardle (Macquarie University), Farhad Yusef-Zadeh (Northwestern University)

The recent identification of one or two sub-parsec disks of young, massive stars orbiting the ~4e6 solar mass black hole Sgr A* has prompted an "in-situ" scenario for star formation in disks of gas formed from a cloud captured from the Galactic centre environment. To date there has been no explanation given for the low angular momentum of the disks relative to clouds passing close to the centre. Here we show that the partial accretion of extended Galactic centre clouds, such as the 50 km/s giant molecular cloud, that temporarily engulf Sgr A* during their passage through the central region of the Galaxy provide a natural explanation for the angular momentum and surface density of the observed stellar disks. The captured cloud material is gravitationally unstable and forms stars as it circularises, potentially explaining the large eccentricity and range of inclinations of the observed stellar orbits. The application of this idea to the formation of the circumnuclear ring is also discussed.

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Milky Way's giant black hole awoke from slumber 300 years ago
A team of Japanese astronomers using ESAs XMM-Newton, along with NASA and Japanese X-ray satellites, has discovered that our galaxys central black hole let loose a powerful flare three centuries ago.
The finding helps resolve a long-standing mystery: why is the Milky Ways black hole so quiescent? The black hole, known as Sagittarius A-star (A*), is a certified monster, containing about 4 million times the mass of our Sun. Yet the energy radiated from its surroundings is thousands of millions of times weaker than the radiation emitted from central black holes in other galaxies.

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Title: Polarised NIR and X-ray Flares from SgrA*
Authors: A. Eckart, F.K. Baganoff, M. Zamaninasab, M. Morris, R. Schoedel, L. Meyer, K. Muzic, M.W. Bautz, W.N. Brandt, G.P. Garmire, G.R. Ricker, D. Kunneriath, C. Straubmeier, W. Duschl, M. Dovciak, V. Karas, S. Markoff, F. Najarro, J. Mauerhan, J. Moultaka, A. Zensus

Stellar dynamics indicate the presence of a super massive 3-4x10^6 Msun solm black hole at the Galactic Centre. It is associated with the variable radio, near-infrared, and X-ray counterpart Sagittarius A* (SgrA*). The goal is the investigation and understanding of the physical processes responsible for the variable emission from SgrA*. The observations have been carried out using the NACO adaptive optics (AO) instrument at the European Southern Observatory's Very Large Telescope (July 2005, May 2007) and the ACIS-I instrument aboard the Chandra X-ray Observatory (July 2005). We find that for the July 2005 flare the variable and polarised NIR emission of SgrA* occurred synchronous with a moderately bright flare event in the X-ray domain with an excess 2 - 8 keV luminosity of about 8x10³³erg/s. We find no time lag between the flare events in the two wavelength bands with a lower limit of less than 10 minutes. The May 2007 flare shows the highest sub-flare to flare contrast observed until now. It provides evidence for a variation in the profile of consecutive sub-flares. We confirm that highly variable and NIR polarised flare emission is non-thermal and that there exists a class of synchronous NIR/X-ray flares. We find that the flaring state can be explained via the synchrotron self-Compton (SSC) process involving up-scattered sub-millimetre photons from a compact source component. The observations can be interpreted in a model involving a temporary disk with a short jet. In the disk component the flux density variations can be explained due to spots on relativistic orbits around the central super massive black hole (SMBH). The profile variations for the May 2007 flare are interpreted as a variation of the spot structure due to differential rotation within the disk.

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Title: Simultaneous Chandra, CSO and VLA Observations of Sgr A*: The Nature of Flaring Activity
Authors: F. Yusef-Zadeh, M. Wardle, C. Heinke, C. D. Dowell, D. Roberts, F. K. Baganoff, G. C. Bower

Sgr A*, the massive black hole at the centre of the Galaxy, varies in radio through X-ray emission on hourly time scales. The flare activity is thought to arise from the innermost region of an accretion flow onto Sgr A*. We present simultaneous light curves of Sgr A* in radio, sub-mm and X-rays that show a possible time delay of 110 ±17 minutes between X-ray and 850 µ m suggesting that the sub-mm flare emission is optically thick. At radio wavelengths, we detect time lags of of 20.4 ± 6.8, 30 ±12 and 20 ±6 minutes between the flare peaks observed at 13 and 7 mm in three different epochs using the VLA. Linear polarisation of 1± 0.2% and 0.7 ±0.1% is detected at 7 and 13 mm, respectively, when averaged over the entire observation on 2006 July 17. A simple picture of an expanding bubble of synchrotron emitting hot plasma can explain the time delay between various wavelengths, the asymmetric shape of the light curves, and the observed polarization of the flare emission at 43 and 22 GHz. The derived physical quantities that characterise the flare emission give the blob expansion speed of v_{exp} ~ 0.003-0.1c, magnetic field of B ~ 10-70 Gauss and particle spectral index p ~ 1-2. These parameters suggest that the expanding gas can not escape from Sgr A* unless it has a large bulk motion.

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