An extraordinary outburst produced by a black hole in a nearby galaxy has provided direct evidence for a population of old, volatile（不稳定的） stellar black holes. The discovery, made by astronomers using NASA's Chandra X-ray Observatory, provides new insight into the nature of a mysterious class of black holes that can produce as much energy in X-rays as a million suns radiate at all wavelengths. Researchers used Chandra to discover a new ultraluminous X-ray source, or ULX. These objects give off more X-rays than most binary systems, in which a companion star orbits the remains of a collapsed star. These collapsed stars form either a dense core called a neutron star or a black hole. The extra X-ray emission suggests ULXs contain black holes that might be much more massive than the ones 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, provides direct evidence that ULXs can contain much older black holes and some sources may have been misidentified as young ones.

The intriguing new ULX is located in M83, a spiral galaxy about 15 million light years from Earth, discovered in 2010 with Chandra. Astronomers compared this data with Chandra images from 2000 and 2001, which showed the source had increased in X-ray brightness by at least 3,000 times and has since become the brightest X-ray source in M83.

The sudden brightening of the M83 ULX is one of the largest changes in X-rays ever seen for this type of object, which do not usually show dormant periods. No sign of the ULX 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," said Roberto Soria of Curtin University in Australia, who led the new study. The dramatic jump in X-ray brightness, according to the researchers, likely occurred because of a sudden increase in the amount of material falling into the black hole.

In 2011, Soria and his colleagues used optical images from the Gemini Observatory and NASA's Hubble Space Telescope to discover a bright blue source at the position of the X-ray source. The object had not been previously observed in a Magellan Telescope image taken in April 2009 or a Hubble image obtained in August 2009. The lack of a blue source in the earlier images indicates the black hole's companion star is fainter, redder and has a much lower mass than most of the companions that previously have been directly linked to ULXs. The bright, blue optical emission seen in 2011 must have been caused by a dramatic accumulation（累积） of more material from the companion star.