Southampton astronomers have been involved in the discovery of a black hole on a massive binge in our nearest neighbour galaxy, Andromeda, which has revealed a new side to the mysterious class of “ultraluminous X-ray sources”.
When black holes feast, they generally emit copious X-ray emissions, but ultraluminous X-ray sources can be bright enough in the X-ray band to outshine their entire galaxy. Astronomers have spent years debating between two main scenarios: either these are unusually massive black holes feeding at a relatively modest level off gas from an orbiting companion star, or else they are rather run-of-the-mill black holes being somehow force fed from the same source.
Using a suite of Earth-orbiting X-ray telescopes, including NASA’s Swift and the European Space Agency’s XMM-Newton satellites, a large international team of astronomers including Professors Rob Fender, Phil Charles and Doctor Anna Scaife, from the University of Southampton Astronomy Group, watched as the X-ray emission from a black hole in the Andromeda galaxy – over two million light years away – brightened and faded dramatically over the course of six months. This type of cycle, while still rare, is also seen when “ordinary” black holes in our Milky Way galaxy are suddenly provided with a free meal. In the process, black holes also launch powerful beams of magnetized plasma called jets, which blast outwards at speeds near that of light, and which emit radio waves.
The discovery of such radio waves indicate that this ultraluminous X-ray source, and by extension, many others, is just a normal, everyday black hole about ten times the mass of the sun, swallowing as much material as it possibly can.
Professor Rob Fender, Head of the Astronomy Group and a co-author on the paper, says: “For the first time, these observations have shown that a bright transient black hole in another galaxy is behaving just like those we see closer to home in our own galaxy. The radio-emitting jets carry away an enormous amount of kinetic energy, re-energising the environment around the black hole even as matter is swallowed up across the event horizon.”
The team trained the US National Science Foundation’s Karl G. Jansky Very Large Array on the black hole, and detected extremely luminous radio emission that dropped by a factor of two in just half an hour. This surprising finding was confirmed by zooming in using the world’s most eagle-eyed radio telescope, the Very Long Baseline Array and was monitored extensively by the UK’s own radio array, AMI-LA in Cambridgeshire. This is the first time that radio jets have been detected from a stellar-mass black hole outside our own galaxy.
Despite the large distance to Andromeda, the absence of dust and gas in that direction allows an unhindered view of the feast, giving scientists key new insights into how jets are produced by a ravenous black hole.
The research, which was published in today’s edition of the scientific journal Nature (http://www.nature.com/nature/journal/v490/n7418/full/nature11490.html) , was funded in the UK by the Science and Technology Facilities Council (STFC).