U-M team finds evidence of elusive intermediate-mass black hole

A U-M team's finding of peculiar X-ray outbursts coming from a black hole indicate that the object has a mass of about 10,000 suns, which would make it part of a new class of black holes.

This composite X-ray/optical image of the spiral galaxy M74 highlights an ultraluminous X-ray source (ULX) shown in the box. ULX sources are distinctive because they radiate 10 to 1,000 times more X-ray power than neutron stars and stellar mass black holes. Chandra observations of this ULX have provided evidence that its X-radiation is produced by a disk of hot gas swirling around a black hole with a mass of about 10,000 suns. (Photo by NASA/CXC/U-M/J. Liu Et Al)

The timing and regularity of these outbursts, observed by a team of U-M researchers at NASA's Chandra X-ray Observatory, make the object one of the best candidates yet for a so-called intermediate-mass black hole.

Scientists have strong evidence for the existence of stellar black holes that are about 10 times as massive as the sun. They also have discovered that supermassive black holes with masses as large as billions of suns exist in the centers of most galaxies. Recent evidence has suggested that a new class of black holes may exist between these extremes—intermediate-mass black holes with masses equal to thousands of suns.

"It is important to verify the existence of intermediate-mass black holes, because they would bridge the gap between stellar-mass black holes and supermassive black holes in the centers of galaxies," says Jifeng Liu a graduate student instructor and research assistant in astronomy and lead author on a paper describing the discoveries that appeared in the March 1 issue of the Astrophysical Journal Letters. "Our observations don't settle the debate, but the behavior of this object is strong evidence in favor of their existence."

Other authors on the research paper are Joel Bregman, professor of astronomy; Ed Lloyd-Davies, a research fellow in astronomy, Jimmy Irwin, assistant research scientist in astronomy; Catherine Espaillat, a doctoral student in astronomy and astrophysics; and Patrick Seitzer, an associate research scientist and lecturer in astronomy—all from U-M.

Liu and his colleagues used Chandra to observe a black hole in the galaxy M74, which is about 32 million light years from Earth in the constellation Pisces. They found that this source exhibits strong, nearly periodic variations in its X-ray brightness every two hours, providing an important clue to the black holes' mass.

The black hole also fell into a class of sources called ultraluminous X-ray sources (ULXs) because they radiate 10 to 1,000 times more X-ray power than neutron stars and stellar mass black holes.

Some astronomers believe these mysterious ULXs are more powerful because they are intermediate-mass black holes. Others think ULXs are regular stellar-mass black holes that appear to be much more powerful in X-rays because their radiation is beamed toward Earth.

The U-M team's discovery of the persistence and long time period of the X-ray variations (called quasi-periodic oscillations, because they are not strictly periodic) of the ULX in M74 is an argument against preferential beaming, Liu says. These variations likely are produced by changes in a disk of hot gas around the black hole. More massive black holes have larger disks, which in turn are expected to vary over longer periods.

Chandra observed M74 in June 2001 and again in October 2001. The XMM-Newton satellite (a European Space Agency mission) also observed this object in February 2002 and January 2003.