An extraordinary X-ray flare is teaching astronomers about a new class of accreting black holes.
The sweeping arms of Messier 83, a grand-design spiral galaxy in the Hydra constellation, are bursting with bright clusters of newly forming stars. Between the arms a rural emptiness prevails, sprinkled with a few dimmer, older stars. But that landscape changed in a December 2010 observation when a mysterious object popped up between the arms, blazing in X-rays and visible light.
The Chandra X-ray Observatory had caught what researcher Robert Soria (Curtin University) and his colleagues call “an unexpected gift” — a new X-ray source so bright that it defies easy explanation. Follow-up observations in visible light revealed a bright new blue object at the exact location of the X-rays.
Even “ordinary” X-ray flare-ups have an exotic explanation. Stellar-mass black holes feed on gas pulled from a companion star, and if that star “burps,” the black hole will get an extra flood of gas. When matter falls down a black hole’s deep gravitational well, it gains immense speed and energy, heating to X-ray temperatures as it jams together to form an accretion disk before finally spiraling into the hole.
But the flare-up in M83 was too powerful to be explained in this way — the X-ray brightness increased by at least 3,000 times over a span of a few years. “Ultraluminous” X-ray sources (ULXs) like the one in M83 exceed the natural limit on how fast gas can fall into a stellar-mass black hole. Beyond this so-called Eddington limit, the incoming gas would become so hot and bright that the pressure from its own radiation would blow it back off. The Eddington limit thus regulates the maximum accretion flow, and hence the maximum luminosity.
The only way out is for ultraluminous sources to have bigger black holes, even though it’s hard to make a very big black hole from a collapsed star. In the case of M83’s ULX, the researchers estimate that the black hole has 40 to 100 times the mass of the Sun. If the black hole were of stellar mass, it would have to be feeding on gas at a rate at least three times the Eddington limit. The blue light seen at the location of the X-ray flare is probably associated with gas flowing into the massive black hole.
But what really interested astronomers was the lack of any trace of a star at the flare-up’s location on earlier images. Most ULXs have young, high-mass companion stars with strong stellar winds to feed the black hole. A young companion star implies that the black hole is similarly young. But any young star would have been bright enough to be observed in images taken before the flare-up, so the donor star for M83’s ULX is likely older. The observations suggest that there are actually two classes of black holes responsible for ULXs. While some black holes are young and growing steadily, an older class of black holes may also exist, growing in flares and spurts.
“ULXs that have old stellar companions must be quite rare and have been elusive,” says Bret Lehmer (Johns Hopkins University), who was not involved in the study. “So I think this is definitely an exciting object that will advance the field.”