European Southern Observatory
18 November 2008
University of Cologne
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Massachusetts Institute of Technology
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Instituto de Astrofísica de Andalucía
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University of Cologne
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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.
Another image of Sag A*, from NASA/Chandra
This Chandra image of Sgr A* and the surrounding region is based on data from a series of observations lasting a total of about one million seconds, or almost two weeks. Such a deep observation has given scientists an unprecedented view of the supernova remnant near Sgr A* (known as Sgr A East) and the lobes of hot gas extending for a dozen light years on either side of the black hole. These lobes provide evidence for powerful eruptions occurring several times over the last ten thousand years. The image also contains several mysterious X-ray filaments, some of which may be huge magnetic structures interacting with streams of energetic electrons produced by rapidly spinning neutron stars. Such features are known as pulsar wind nebulas.
NASA/CXC/MIT/F. Baganoff, R. Shcherbakov et al.
Date 7 January 2010
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.
“Observations like this, over a range of wavelengths, are really the only way to understand what’s going on close to the black hole,” says Andreas Eckart of the University of Cologne, who led the team.
Sagittarius A* is located at the centre of our own Milky Way Galaxy at a distance from Earth of about 26 000 light-years. It is a supermassive black hole with a mass of about four million times that of the Sun. Most, if not all, galaxies are thought to have a supermassive black hole in their centre.
“Sagittarius A* is unique, because it is the nearest of these monster black holes, lying within our own galaxy,” explains team member Frederick K. Baganoff of the Massachusetts Institute of Technology (MIT) in Cambridge, USA. “Only for this one object can our current telescopes detect these relatively faint flares from material orbiting just outside the event horizon.”
The emission from Sagittarius A* is thought to come from gas thrown off by stars, which then orbits and falls into the black hole.
Making the simultaneous observations required careful planning between teams at the two telescopes. After several nights waiting at the two observatory sites, they struck lucky.
“At the VLT, as soon as we pointed the telescope at Sagittarius A* we saw it was active, and getting brighter by the minute. We immediately picked up the phone and alerted our colleagues at the APEX telescope,” says Gunther Witzel, a PhD student from the University of Cologne.
Macarena García-Marín, also from Cologne, was waiting at APEX, where the observatory team had made a special effort to keep the instrument on standby. “As soon as we got the call we were very excited and had to work really fast so as not to lose crucial data from Sagittarius A*. We took over from the regular observations, and were in time to catch the flares,” she explains.
Over the next six hours, the team detected violently variable infrared emission, with four major flares from Sagittarius A* . The submillimetre-wavelength results also showed flares, but, crucially, this occurred about one and a half hours after the infrared flares.
The researchers explain that this time delay is probably caused by the rapid expansion, at speeds of about 5 million km/h, of the clouds of gas that are emitting the flares. This expansion causes changes in the character of the emission over time, and hence the time delay between the infrared and submillimetre flares.
Although speeds of 5 million km/h may seem fast, this is only 0.5% of the speed of light. To escape from the very strong gravity so close to the black hole, the gas would have to be travelling at half the speed of light – 100 times faster than detected – and so the researchers believe that the gas cannot be streaming out in a jet. Instead, they suspect that a blob of gas orbiting close to the black hole is being stretched out, like dough in a mixing bowl, and this is causing the expansion.
The simultaneous combination of the VLT and APEX telescopes has proved to be a powerful way to study the flares at multiple wavelengths. The team hope that future observations will let them prove their proposed model, and discover more about this mysterious region at the centre of our Galaxy.
The members of the international team who did this research are: A. Eckart (University of Cologne, Germany), R. Schödel (Instituto de Astrofísica de Andalucía – CSIC, Spain), M. García-Marín (University of Cologne, Germany), G. Witzel (University of Cologne, Germany), A. Weiss (MPIfR, Germany), F. K. Baganoff (MIT, USA), M. R. Morris (University of California, USA), T. Bertram (University of Cologne, Germany), M. Dovčiak (Astronomical Institute of the Academy of Sciences of the Czech Republic), D. Downes (IRAM, France), W.J. Duschl (Christian-Albrechts-Universität, Germany), V. Karas (Astronomical Institute of the Academy of Sciences of the Czech Republic), S. König (University of Cologne, Germany), T. P. Krichbaum (MPIfR, Germany), M. Krips (Harvard-Smithsonian Center for Astrophysics, USA), D. Kunneriath (University of Cologne, Germany), R.-S. Lu (MPIfR, Germany), S. Markoff (Astronomical Institute ‘Anton Pannekoek’, Netherlands), J. Mauerhan (University of California, USA), L. Meyer (University of California, USA), J. Moultaka (LATT, France), K. Mužić (University of Cologne, Germany), F. Najarro (Centro de Astro Biologia, Madrid, Spain), J.-U. Pott (University of California, USA), K. F. Schuster (IRAM, France), L. O. Sjouwerman (NRAO, USA), C. Straubmeier (University of Cologne, Germany), C. Thum (IRAM, France), S. Vogel (University of Maryland, USA), H. Wiesemeyer (IRAM, Spain), M. Zamaninasab (University of Cologne, Germany), J. A. Zensus (MPIfR, Germany)
[My purpose in listing the complete team as rendered by ESO was to exhibit that there are U.S. scientists able to get on these explorations by attaching to a team which is primarily from countries which do support ESO. It is possible, but, I say, it is probably rare.]
See the full article, with notes, here.
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