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Synchronised observations catch flares from

Sagittarius A*
BY DR EMILY BALDWIN
ASTRONOMY NOW

Posted: 19 November, 2008

Simultaneous observations made with the VLT and APEX telescopes have revealed the nature of four violent flares emanating from the centre of our Milky Way Galaxy.

Astronomers used ESO’s Very Large Telescope (VLT) and the Atacama Pathfinder Experiment (APEX) to study the region around the Milky Way’s black hole, known as Sagittarius A*, at near-infrared and sub-millimetre wavelengths respectively. Sagittarius A* is a supermassive black hole lurking at the centre of the Milky Way, and boasts a mass of about four million times that of the Sun. Emission from the galactic monster is thought to come from gas thrown off by proximal stars via a strong stellar wind, which then orbits and falls into the jaws of the black hole. Around four to six flares are observed from Sagittarius A* at infrared wavelengths every day.

Colour composite image of the central region of our Milky Way galaxy, about 26,000 light years from Earth. Giant clouds of gas and dust are shown in blue, as detected by the LABOCA instrument on the APEX telescope at sub-millimetre wavelengths. The image also contains near-infrared data from the 2MASS project at K-band (in red), H-band (in green), and J-band (in blue). The image shows a region approximately 100 light-years wide. Image: ESO/APEX/2MASS/A. Eckart et al.

"Only for this one object can our current telescopes detect these relatively faint flares from material orbiting just outside the event horizon," says Frederick Baganoff of the Massachusetts Institute of Technology (MIT). The event horizon marks the boundary beyond which no matter or radiation can escape the gravitational field of the black hole. While supermassive black holes are thought to exist in all galaxies, Sagittarius A* is the nearest one that astronomers can study in such detail.

Making the simultaneous observations required careful planning between teams at the two locations, and is the first time that astronomers have caught a flare by synchronising their observations in this way. "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 lead astronomer Andreas Eckart of the University of Cologne.

Over a period of six hours the VLT team detected extremely variable emission at infrared wavelengths, with four major flares erupting from Sagittarius A* in that time. The sub-millimetre wavelength results also showed flares, but crucially, they occurred about one and a half hours after the infrared flares. The astronomers put this down to the fact that the clouds of gas that are emitting the flares are expanding rapidly at rates of five million kilometres per hour. This expansion causes changes in the character of the emission over time, resulting in the time delay between the infrared and sub-millimetre flares.

Artist impression of a bright blob of gas in the disc of material surrounding the black hole in the centre of our Galaxy, Sagittarius A* and responsible for the flares detected in this study. As the blob orbits the black hole, it is stretched out, and this expansion over time causes the delay between flares being detected at near-infrared wavelengths (with the VLT) and at sub-millimetre wavelengths (with APEX). Image: ESO/L. Calçada.

The speed of the expansion, while fast, is actually only 0.5 percent of the speed of light, and provides more insight into the nature and cause of the flares. In order to escape from the immensely strong gravity that surrounds the black hole, the gas would have to be travelling at half the speed of light – 100 times faster than that detected – and so the researchers conclude that the gas cannot be streaming out in a jet from the black hole itself. Instead, they suspect that a ‘blob’ of gas orbiting close to the black hole is being stretched out and this is causing the expansion, and therefore the time delay between flares being detected by the VLT and APEX.

By simultaneously carrying out observations at different wavelengths, the astronomers have created a powerful tool to reveal the nature of flares emitted in the regions of black holes. As a next step, Eckart and his team hope to make further observations that will allow them to develop their proposed model for flare emission, and discover more about this mysterious region at the centre of our Galaxy.

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