A population of old, volatile stellar black holes has been discovered, following an 'extraordinary outburst' from one located in a nearby galaxy.
Astronomers using NASA's Chandra X-ray Observatory have found a new ultraluminous X-ray source, or ULX. These objects - either black holes or neutron stars - give off more X-rays than most binary systems, indicating they may contain black holes that are much more massive than those found elsewhere in our galaxy.
The companion stars to ULXs, when identified, are usually young, massive stars, implying their black holes are also young. The latest research, however, shows that ULXs can contain much older black holes, and that some may have been misidentified as young ones.
The highly-active new ULX is located in M83, a spiral galaxy about 15 million light years from Earth, discovered using Chandra in 2010. Comparing this data with Chandra images from 2000 and 2001 showed that it's become at least 3,000 times brighter, making it the brightest X-ray source in M83.
The sudden brighteningis one of the biggest changes in X-rays ever seen for this type of object, which don't usually go through dormant periods.
But no sign of it was found in historical X-ray images made with Einstein Observatory in 1980, ROSAT in 1994, the European Space Agency's XMM-Newton in 2003 and 2008, or NASA's Swift observatory in 2005.
"The flaring up of this ULX took us by surprise and was a sure sign we had discovered something new about the way black holes grow," says Roberto Soria of Curtin University in Australia.
The team believes it probably happened because of a sudden increase in the amount of material falling into the black hole from its companion star.
This companion is likely a red giant star at least 500 million years old, with a mass almost four times the sun's. Theoretical models for the evolution of stars suggest the black hole should be almost as old as its companion.
Another ULX containing a volatile, old black hole has also recently been discovered in the Andromeda galaxy by Amanpreet Kaur, from Clemson University, and colleagues. It's highly variable and its companion is an old, red star.
"With these two objects, it's becoming clear there are two classes of ULX, one containing young, persistently growing black holes and the other containing old black holes that grow erratically," says Kip Kuntz of Johns Hopkins University.
"We were very fortunate to observe the M83 object at just the right time to make the before and after comparison."