* Astronomy

Stellar Orbits in the Central Parsec.


Image created by Prof. Andrea Ghez and her research team at UCLA and are from data sets obtained with the W. M. Keck Telescopes

A 2.2 micron image of the stellar orbits in the central parsec. Images taken from the years 1995 through 2006 are used to track specific stars orbiting the proposed black hole at the center of the Galaxy. These orbits, and a simple application of Kepler's Laws, provide the best evidence yet for a supermassive black hole, which has a mass of 4 million times the mass of the Sun. Especially important are the stars S0-2, which has an orbital period of only 15.56 years, and S0-16, which comes a mere 90 astronomical units from the black hole.

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Title: K-band polarimetry of an Sgr A* flare with a clear sub-flare structure
Authors: L. Meyer, R. Schoedel, A. Eckart, V. Karas, M. Dovciak, W. J. Duschl

Context: The supermassive black hole at the Galactic centre, SgrA*, shows frequent radiation outbursts, often called 'flares'. In the near-infrared some of these flares were reported as showing intrinsic quasi-periodicities. The flux peaks associated with the quasi-periodic behaviour were found to be highly polarized. Aims: The aim of this work is to present new evidence to support previous findings of the properties of the polarized radiation from SgrA* and to again provide strong support for the quasi-periodicity of ~18±3 min reported earlier.
Methods: Observations were carried out at the European Southern Observatory's Very Large Telescope on Paranal, Chile. We used the NAOS/CONICA adaptive optics/near-infrared camera instrument. By fitting the polarimetric lightcurves with a hot-spot model, we addressed the question of whether the data are consistent with this model. To fit the observed data we used a general relativistic ray-tracing code in combination with a simple hot-spot/ring model.
Results: We report on new polarisation measurements of a K-band flare from the supermassive black hole at the Galactic centre. The data provide very strong support for a quasi-periodicity of 15.5 ±2 min. The mean polarisation of the flare is consistent with the direction of the electric field vector that was reported in previous observations. The data can be modelled successfully with a combined blob/ring model. The inclination i of the blob orbit must be i > 20 deg on a 3sigma level, and the dimensionless spin parameter of the black hole is derived to be a > 0.5.

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Title: Sub-milliarcsecond Imaging of SgrA* and M87
Authors: T.P. Krichbaum, D.A. Graham, M. Bremer, W. Alef, A. Witzel, J.A. Zensus, A. Eckart

Researchers present and discuss new result from mm-VLBI observations of M87 and SgrA*, using the Global mm-VLBI array (GMVA).
New 3mm-VLBI images of the inner jet of M87 are presented, showing details with a spatial resolution down to 15 Schwarzschild radii. This resolution corresponds to a similar spatial resolution (in terms of R_s) obtained for Sgr A*.
They discuss existing and new size determinations for this source provided by mm-VLBI at wavelengths of 3, 2, and 1.4 mm. With respect to the morphological difference between Sgr A* and M87 (the latter exhibits a long jet), a comparison of the sub-milliarcsecond structures seen with mm-VLBI may lead to a better understanding of the astro-physical processes acting in the vicinity of super-massive black holes.

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In the most comprehensive study of Sagittarius A* (Sgr A*), the enigmatic supermassive black hole in the centre of the Milky Way Galaxy, astronomers -- using nine ground and space-based telescopes including the Hubble Space Telescope and the XMM-Newton X-ray Observatory -- have discovered that Sgr A* produces rapid flares close to the innermost region of the black hole in many different wavelengths and that these emissions go up and down together.

This insight into the frequent bursts of radiation observed shooting off the black hole like firecrackers -- similar to solar flares -- will help scientists better understand the dynamics of Sgr A* and the source of its flares.
Farhad Yusef-Zadeh, professor of physics and astronomy at North-western University, who led a team of 11 astronomers from around the world in the study of Sgr A*, presented the team’s results at a press conference January 10 at the American Astronomical Society meeting in Washington, D.C.

"We observed that the less energetic infrared flares occur simultaneously with the more energetic X-ray flares as well the submillimeter flares. From this, we infer that the particles that are accelerated near the black hole give rise to X-ray, infrared and submillimeter emission. In addition, not all of the material that approaches the black hole gets sucked in. Some of the material may be ejected from the vicinity of the central black hole or event horizon. Our observations hint that these flares have enough energy to escape from the closest confines of the supermassive black hole’s sphere of influence" - Farhad Yusef-Zadeh.


These images show the flare activity from A* (Sgr A*). Both show Sgr A* when it was quiescent or quiet (left), when it was active (middle) and the difference between them (right), which clearly shows Sgr A* is the only source that is flaring.
Credit Northwestern University

Yusef-Zadeh and his team observed Sgr A* during two four-day periods in 2004, one in March and one in September. (2004 marked the 30th anniversary of the discovery of Sgr A*, which has a mass equivalent to 3.6 million Suns and is located in the Sagittarius constellation.) The campaign captured data across a wide spectrum, including radio, millimetre, submillimeter, infrared, X-ray and soft gamma ray wavelengths.
The astronomers also determined that the real engine of the flare activity is in the infrared wavelength. Using observations from Hubble’s Near-Infrared Camera and Multi-Object Spectrometer, they found infrared activity 40 percent of the time, more than was observed at any other wavelength.

"This is not something we expected. Other black holes in other galaxies don’t show this flare activity. We believe it is the dynamics of the captured material -- very close to the event horizon of the black hole -- that produces the flares. And the flares are fluctuating at low levels, like flickers. The flare radiation results from fast-moving materials in the innermost region of the black hole. It’s a way of life for Sgr A*, this frequent low level of activity" - Farhad Yusef-Zadeh.

Because flares are variable and not constant, the study required a large number of telescopes devoted to studying flare activity simultaneously. The space-based telescopes used in this observation campaign were the Hubble Space Telescope, the XMM-Newton X-ray Observatory and the International Gamma-Ray Astrophysics Laboratory (INTEGRAL). The ground telescopes used were Very Large Array (VLA) of the National Radio Astronomy Observatory; Caltech Submillimeter Observatory (CSO); Submillimeter Telescope (SMT); Nobeyama Array (NMA); Berkeley Illinois Maryland Array (BIMA); and Australian Telescope Compact Array (ATCA).

In addition to Yusef-Zadeh, the Sgr A* study was conducted by Howard A. Bushouse, Space Telescope Science Institute; Mark Wardle, Macquarie University, Australia; Douglas Roberts, Northwestern University and Adler Planetarium and Astronomy Museum; Craig Heinke, Northwestern University; C. Darren Dowell, Jet Propulsion Laboratory, California Institute of Technology; Geoffrey Bower, University of California, Berkeley; Stuart Shapiro, University of Illinois at Urbana-Champaign; Andrea Goldwurm and Guillaume Belanger, Service d’Astrophysique, France; and Baltasar Vila-Vilaro, National Astronomical Observatory of Japan.

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