Tuesday, October 30, 2012

Looks like Lunch for Sag A*?

In the past week I've fielded several questions about the gas cloud, G2, which is possibly on course to be accreted by the supermassive black hole at the centre of our Galaxy. So I decided I would update my knowledge (and the wikipedia page about Sag A*) on this object with the below:

Discovery of G2 Gas Cloud on an Accretion Course with Sag A*

An artists impressive of G2 approaching Sag A* (orange). The blue lines indicate orbits of known stars about the black hole. Credit: ESO/MPE/Marc Schartmann
First noticed as something unusual in images of the centre of our Galaxy in 2002 (https://www.llnl.gov/news/newsreleases/2012/Oct/NR-12-10-07.html), 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 Sag A* in a paper published by Nature in 2012 (Gillessen et al. 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 impact this might have on both G2 and the black hole. G2 appears to already be being distrupted over the past 3 years of observation (Gillessen et al. 2012), and may be completely destroyed by the encounter. If this is the case a significant amount of it may be accreted by Sag 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 (http://www.scientificamerican.com/article.cfm?id=black-hole-gas-blob) 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 Sag A* and the ensemble may pass by without any effect. 

The average rate of accretion onto Sag A* is unusually small for a black hole of its mass (Morris et al. 2012) 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 (https://wiki.mpe.mpg.de/gascloud/ProposalList). 

Groups at ESO (http://www.eso.org/public/news/eso1151/) and LLNL (https://www.llnl.gov/news/newsreleases/2012/Oct/NR-12-10-07.html have been working on simulations of the passage. 

Gillessen et al. 2012
Morris et al. 2012

No comments:

Post a Comment