NASA scientists have found two smoking-gun features of an intermediate-mass black hole that suggest these newly identified objects are fundamentally different from other types of black holes, running hotter than expected.

The observation further establishes these objects as a new class of black hole, yet offers a perplexing twist: Intermediate-mass black holes do not appear to suck in matter the same way as their larger and smaller cousins do.

Suspected intermediate-mass black holes in galaxy M82. Credit: NASA/SAO/CXC

Drs. Tod Strohmayer and Richard Mushotzky of NASA Goddard Space Flight Center in Greenbelt, Md., present these findings today at a press conference at the meeting of the High Energy Astrophysics Division of the American Astronomical Society at Mt. Tremblant, Quebec. The observation was made with the European Space Agency's XMM-Newton satellite and NASA's Rossi X-ray Timing Explorer.

"We have been neatly assembling pieces of the black-hole puzzle over the years," said Strohmayer. "Now we have new pieces, some of which are familiar and others which don't seem to fit. A picture may emerge in which intermediate-mass black holes are a different beast altogether."

Black holes are objects so dense and have a gravitational potential so heightened that nothing, not even light, can escape the pull if it ventures too close. Although the black holes themselves are invisible, the region surrounding them glow furiously as matter pours in.

Stellar black holes are the remains of massive stars that have imploded, left with the mass of up to about ten suns. Supermassive black holes contain the mass of millions to billions of suns confined to a region about the size of our Solar System. Scientists suspect that intermediate black holes contain the mass of about 500 to 10,000 suns. Stellar and supermassive black holes exhibit similar features, only scaled accordingly. Intermediate-mass black holes might buck this trend, Strohmayer said.

Scientists call intermediate-mass black holes ultra-luminous X-ray sources (ULXs). While they are clearly extremely bright sources of X-ray radiation, these objects could be smaller black holes with all of their energy (or, light) beamed in our direction, like a flashlight shined directly into the eyes. This would make them appear intrinsically brighter (and more massive) than they really are.

Strohmayer and Mushotzky's observation strongly rules out the beaming model for one of the brightest ULXs in galaxy M82. The scientists uncovered, for the first time, a type of flickering in this ULX's X rays called quasiperiodic oscillations (QPOs). The oscillations likely arise from gas whipping around the black hole in a tight and frenzied orbit the collective, periodic motion of material in what is known as a black hole accretion disk. It is unlikely that light from an entire accretion disk would be beamed, Strohmayer said.

The scientists also detected the first "broad iron line" from a ULX. This refers to a pattern of X-ray light from iron atoms stretched by extreme gravity, a telltale sign of black hole shenanigans afoot, as Einstein predicted. The one-two punch of a QPO and broad iron line suggests that this ULX in M82 is a non-beamed black hole at least 50 times more massive than a stellar black hole.

What the scientists cannot explain, however, is why the M82 ULX accretion disk is so hot. Theory predicts that smaller black holes have hotter accretion disks, particularly in the inner ring closest to the black hole. This is because material can swirl faster and more closely around a stellar size black hole compared to a supermassive black hole. Yet, according to XMM-Newton data, the intermediate-sized black hole in M82 has an accretion disk hotter than those found around stellar black holes.

"Something new and exotic may be taking place in this object to heat the accretion disk to such high temperatures," said Strohmayer. "The nature of these objects is one of the most interesting conundrums in high energy astrophysics."

XMM-Newton was launched from French Guiana in December 1999 and carries three advanced X-ray telescopes. NASA Goddard hosts the XMM U.S. guest visitor support center. NASA Goddard operates the Rossi Explorer, which was launched in 1995.