* Astronomy

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RE: Sagittarius A*

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Galactic Center.kmz
Google Sky File

-- Edited by Blobrana at 14:56, 2008-01-12

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The central region of our Milky Way is an extremely interesting and fascinating field of research. Within few light years we find here ten thousands of stars forming a dense cluster, and the geometric centre of our Galaxy harbours a supermassive black hole with around 3.6 million solar masses. Due to its relative proximity of around 8 kiloparsecs, the Galactic Centre is a perfect laboratory to examine the physical processes in a galactic nucleus.

Via Max-Planck-Institut für extraterrestrische Physik - Infrared/Submillimeter Astronomy - Galactic Centre Research

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Title: The Galactic Centre
Authors: R. Genzel, V. Karas

In the past decade high resolution measurements in the infrared employing adaptive optics imaging on 10m telescopes have allowed determining the three dimensional orbits stars within ten light hours of the compact radio source at the centre of the Milky Way. These observations show the presence of a three million solar mass black hole in Sagittarius A* beyond any reasonable doubt. The Galactic Centre thus constitutes the best astrophysical evidence for the existence of black holes which have long been postulated, and is also an ideal 'lab' for studying the physics in the vicinity of such an object. Remarkably, young massive stars are present there and probably have formed in the innermost stellar cusp. Variable infrared and X-ray emission from Sagittarius A* are a new probe of the physical processes and space-time curvature just outside the event horizon.

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Title: A Possible Link Between the Galactic Centre HESS Source and Sgr A*
Authors: D.R. Ballantyne (1), Fulvio Melia (1,2), Siming Liu (3), Roland M. Crocker (4) ((1) Physics Dept., U. of Arizona (2) Steward Observatory (3) LANL (4) Univ. of Adelaide)
(revised v2)

Recently, HESS and other air Cerenkov telescopes have detected a source of TeV gamma-rays coincident with the Galactic centre. It is not yet clear whether the gamma-rays are produced via leptonic or hadronic processes, so it is important to consider possible acceleration sites for the charged particles which produce the gamma-rays. One exciting possibility for the origin of these particles is the central black hole, Sgr A*, where the turbulent magnetic fields close to the event horizon can accelerate protons to TeV energies. Using a realistic model of the density distribution in a 6 pc x 6 pc x 6pc cube at the Galactic centre, we here calculate the trajectories followed by these TeV protons as they gyrate through the turbulent medium surrounding Sgr A*. Diffusing out from the black hole, the protons produce TeV gamma-rays via pi^0 decay following a collision with a proton in the surrounding medium. After following over 222,000 such trajectories, we find that the circumnuclear ring around Sgr A* can reproduce the observed 0.1-100 TeV HESS spectrum and flux if the protons are injected into this medium with an effective power-law index of 0.75, significantly harder than the observed photon index of 2.25. The total energy in the steady-state 1-40 TeV proton population surrounding Sgr A* is inferred to be approx 5x10^{45} ergs. Only 31% of the emitted 1-100 TeV protons encounter the circumnuclear torus, leaving a large flux of protons that diffuse outward to contribute to the Galactic ridge emission observed by HESS on scales of >~ 1 degree.

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Title: A Possible Link Between the Galactic Centre HESS Source and Sgr A*
Authors: D.R. Ballantyne (1), Fulvio Melia (1,2), Siming Liu (3), Roland M. Crocker (4) ((1) Physics Dept., U. of Arizona (2) Steward Observatory (3) LANL (4) Univ. of Adelaide)

Recently, HESS and other air Cherenkov telescopes have detected a source of TeV gamma-rays coincident with the Galactic centre. It is not yet clear whether the gamma-rays are produced via leptonic or hadronic processes, so it is important to consider possible acceleration sites for the charged particles which produce the gamma-rays. One exciting possibility for the origin of these particles is the central black hole, Sgr A*, where the turbulent magnetic fields close to the event horizon can accelerate protons to TeV energies. Using a realistic model of the density distribution in a 6 pc x 6 pc x 6pc cube at the Galactic centre, we here calculate the trajectories followed by these TeV protons as they gyrate through the turbulent medium surrounding Sgr A*. Diffusing out from the black hole, the protons produce TeV gamma-rays via pi^0 decay following a collision with a proton in the surrounding medium. After following over 222,000 such trajectories, we find that the circumnuclear ring around Sgr A* can reproduce the observed 0.1-100 TeV HESS spectrum and flux if the protons are injected into this medium with an effective power-law index of 0.75, significantly harder than the observed photon index of 2.25. The total energy in the steady-state 1-40 TeV proton population surrounding Sgr A* is inferred to be approx 2x10^{47} ergs. Only 31% of the emitted 1-100 TeV protons encounter the circumnuclear torus, leaving a large flux of protons that diffuse outward to contribute to the Galactic ridge emission observed by HESS on scales of >~ 1 degree.

Read more (180kb, PDF)

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ESA's gamma ray observatory Integral has caught the centre of our galaxy in a moment of rare quiet.

ESA's gamma ray observatory Integral has caught the centre of our galaxy in a moment of rare quiet. A handful of the most energetic high-energy sources surrounding the black hole at the centre of the Galaxy had all faded into a temporary silence when Integral looked.
This unusual event is allowing astronomers to probe for even fainter objects and may give them a glimpse of matter disappearing into the massive black hole at the centre of our galaxy. Integral has discovered many new sources of high-energy radiation near the galactic centre. From February 2005, Integral began to regularly monitor the centre of the Galaxy, and its immediate environment, known as the Galactic bulge.
Erik Kuulkers of ESA's Integral Science Operations Centre, ESAC, Spain, leads the Galactic bulge monitoring programme. Integral now keeps its high-tech eyes on about 80 high-energy sources in the galactic bulge.

"Most of these are X-ray binaries" - Erik Kuulkers.

Expand (118kb, 1059 x 589)
Credits: ESA/ISDC

According to the Integral observations in April 2006, the high-energy rays from about ten sources closest to the galactic centre all faded temporarily. The fortuitous dimming allows astronomers to set new limits on how faint these X-ray binaries can become. It also allows a number of new investigations to be undertaken with the data.

"When these normally bright sources are faint, we can look for even fainter sources" - Erik Kuulkers .

These could be other X-ray binaries or the high-energy radiation from giant molecular clouds interacting with past supernovae. There is also the possibility of detecting the faint high-energy radiation from the massive black hole in our Galaxy's centre.
Integral's Galactic bulge monitoring programme will continue throughout this year. The data is made available, within a day or two of being collected, to the scientific community via the Internet from a dedicated webpage at the Integral Science Data Centre (IDSC), Geneva, Switzerland. This way, anyone interested in specific sources can watch for interesting changes and trigger follow up observations with other telescopes in good time.

The findings are accepted for publication in the Astronomy & Astrophysics magazine, in the article titled: "The INTEGRAL Galactic bulge monitoring program: the first 1.5 years", by E.Kuulkers et al. Astronomers from the Danish National Space Centre participated in the program with data from the JEM-X instrument.

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This set of Chandra images shows evidence for a light echo generated by the Milky Way's supermassive black hole, a.k.a. Sagittarius A* (pronounced "A-star"). Astronomers believe a mass equivalent to the planet Mercury was devoured by the black hole about 50 years earlier, causing an X-ray outburst which then reflected off gas clouds near Sagittarius A*.
The large image shows a Chandra view of the middle of the Milky Way, with Sagittarius A* labelled. The smaller images show close-ups of the region marked with ellipses. Clear changes in the shapes and brightness of the gas clouds are seen between the 3 different observations in 2002, 2004 and 2005. This behaviour agrees with theoretical predictions for a light echo produced by Sagittarius A* and helps rule out other interpretations.

Credit: NASA/CXC/Caltech/M.Muno et al

Position(2000): RA 17h 45m 58.013s Dec -28º 56' 33.11"

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-- Edited by Blobrana at 19:33, 2007-01-10

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Title: Flaring Activity of Sgr A*: Expanding Hot Blobs
Authors: F. Yusef-Zadeh, M. Wardle, D. A. Roberts, C. O. Heinke, C. D. Dowell, W. D. Cotton, G. C. Bower, F. K. Baganoff

Sgr A* is considered to be a massive black hole at the Galactic center and is known to be variable in radio, millimeter, near-IR and X-rays. Recent multi-wavelength observing campaigns show a simultaneous X-ray and near-IR flare, as well as sub-millimetre and near-IR flares from Sgr A*. The flare activity is thought to be arising from the innermost region of Sgr A*. We have recently argued that the duration of flares in near-IR and submillimeter wavelengths implies that the burst of emission expands and cools on a dynamical time scale before the flares leave Sgr A*. The detection of radio flares with a time delay in the range of 20 and 40 minutes between 7 and 12mm peak emission implies adiabatic expansion of a uniform, spherical hot blob due to flare activity. We suspect that this simple outflow picture shows some of the characteristics that are known to take place in microquasars, thus we may learn much from comparative study of Sgr A* and its environment vs. microquasars.

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