Astronomers from the University of Maryland, College Park (UMCP) and NASA's Goddard Space Flight Center have uncovered rhythmic pulsations from a rare breed of black hole in archival data from NASA's Rossi X-ray Timing Explorer (RXTE) satellite.
The signals provide compelling evidence that the object, known as M82 X-1, is one of only a few midsize black holes known.
Dying stars form modest black holes measuring up to around 25 times the mass of our sun. At the opposite extreme, most large galaxies contain a supermassive black hole with a mass tens of thousands of times greater. Just as drivers traveling a highway packed with compact cars and monster trucks might start looking for sedans, astronomers are searching for a middle range of the black hole population and wondering why they see so few.
M82 X-1 is the brightest X-ray source in Messier 82, a galaxy located about 12 million light-years away in the constellation Ursa Major. While astronomers have suspected the object of being a midsize, or intermediate-mass, black hole for at least a decade, estimates have varied from 20 to 1,000 solar masses, preventing a definitive classification.
Working with Mushotzky and Strohmayer, UMCP graduate student Dheeraj Pasham sifted through about 800 RXTE observations of M82 in a search for specific types of brightness changes that would help pin down the mass of the X-ray source.
As gas streams toward the black hole it piles up into a disk around it. Friction within the disk heats the gas to millions of degrees, which is hot enough to emit X-rays. Cyclical intensity variations in these X-rays reflect processes occurring within the disk.
Scientists think the most rapid changes occur near the inner edge of the disk on the brink of the black hole's event horizon, the point beyond which nothing, not even light, can escape. With such close proximity to the black hole, the effects of Einstein's general relativity come into play, resulting in X-ray variations that repeat at nearly regular intervals.
Astronomers call these signals quasi-periodic oscillations, or QPOs, and have shown that for black holes produced by stars, their frequencies scale up or down depending on the size of the black hole.
When astronomers study X-ray fluctuations from many stellar-mass black holes, they see both slow and fast QPOs, but the fast ones often come in pairs with a specific 3:2 rhythmic relationship. For every three flashes from one member of the QPO pair, its partner flashes twice.
The combined presence of slow QPOs and a faster pair in a 3:2 rhythm effectively sets a standard scale that gives scientists a powerful tool for establishing the masses of stellar black holes.
A decade ago, Strohmayer and Mushotzky showed the presence of slow QPO signals from M82 X-1. In order to apply the tried-and-true relationship used for stellar-mass black holes, the researchers needed to identify a pair of steady fluctuations exhibiting the same 3:2 beat in RXTE observations. By analyzing six years of data, they located X-ray variations that reliably repeated about 3.3 and 5.1 times each second, just the 3:2 relationship they needed.
This allowed them to calculate that M82 X-1 weighs about 400 solar masses -- the most accurate determination to date for this object and one that clearly places it in the category of intermediate-mass black holes.