Discovery of G2 Gas Cloud On An Accretion Course
First noticed as something unusual in images of the centre of our Galaxy in 2002, the gas cloud, G2, which has has mass about 3 times that of the Earth was confirmed to be likely on a course taking it into the accretion zone of Sgr A* in a paper published in Nature in 2012. Predictions of its orbit suggest it will have a closest approach to the black hole (a perinigricon) in mid to late 2013. At this time the gas cloud will be at a distance of just over 3000 times the radius of the event horizon (or ~260 AU, 36 light hours) from the black hole. Opinions differ as to the effect this might have on both G2 and the black hole. G2 appears to already be being distrupted over the past 3 years of observation, and may be completely destroyed by the encounter. If this is the case a significant amount of it may be accreted by Sgr A* which could lead to a significant brightening of X-ray and other emission from the black hole, likely to last over the next several decades. Other astronomers have suggested the gas cloud may be hiding a dim star, or even a stellar mass black hole, which would hold it together against the tidal forces of Sgr A* and the ensemble may pass by without any effect.
The average rate of accretion onto Sgr A* is unusually small for a black hole of its mass and is only detectable because it's so close to us. This passage of G2 in 2013 will offer astronomers the chance to learn a lot more about how material accretes onto supermassive black holes. A suite of astronomical facilities are planning to observe this closest approach, with observations confirmed with Chandra, XMM, EVLA, INTEGRAL, Swift, Fermi and requested at VLT and Keck.
Groups at ESO ( link to press release) and LLNL (link to press release) have been working on simulations of the passage.
Read more about this topic: Sagittarius A*
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