Black holes are one of the most unusual and fascinating phenomena in nature, and now astronomers are learning more about a peculiar type of black hole, thanks to archived data from NASA’s Rossi X-ray Timing Explorer (RXTE) satellite. The black hole, called M82 X-1, is about 12 million light-years away and one of only a few known medium-sized black holes.
Most black holes come in two sizes, either “small” (about 25 times the mass of our Sun) or enormous monsters tens of thousands of times more massive than the Sun. Relatively few are in between those two extremes.
“Between the two extremes of stellar and supermassive black holes, it’s a real desert, with only about half a dozen objects whose inferred masses place them in the middle ground,” said Tod Strohmayer, an astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Md.
M82 X-1 is the brightest X-ray source in the galaxy Messier 82. It was suspected of being a mid-sized black hole for at least a decade, but now the new evidence backs that idea up, as stated by astronomers from Goddard and the University of Maryland, College Park (UMCP). According to Richard Mushotzky, a professor of astronomy at UMCP, “For reasons that are very hard to understand, these objects have resisted standard measurement techniques.”
As it turns out, M82 X-1 weighs in at approximately 400 solar masses. Astronomers discovered specific changes in brightness in the black hole, which enabled them to better calculate its size.
While black holes can’t be seen directly, gasses which fall into them emit X-rays which can be observed. Variations in the brightness occur depending on how close the gasses are to the black hole; gasses which are closest to the event horizon, the point beyond which not even light can escape, fluctuate the most rapidly. These rhythmic pulsations are called quasi-periodic oscillations, or QPOs.
“When we study fluctuations in X-rays from many stellar-mass black holes, we see both slow and fast QPOs, but the fast ones often come in pairs with a specific 3:2 rhythmic relationship,” explained Dheeraj Pasham, UMCP graduate student.
Six years of RXTE data were combed through to come up with the new results, which verified the 3:2 rhythmic relationship; the X-ray variations reliably repeated about 5.1 and 3.3 times a second, which fit that pattern nicely.
RXTE was launched in 1995 and decommissioned in 2012. There is, however, still a lot of valuable information to be learned from the data collected, as the new study shows. In 2016, a new X-ray mission, the Neutron Star Interior Composition Explorer (NICER), is scheduled to be launched to the International Space Station, which will continue where RXTE left off. Six additional, potentially mid-sized black holes have also now been identified, which NICER should be able to examine more closely.
The results of the new study have been published online in the Aug. 17 issue of the journal Nature. The paper, “A 400-solar-mass black hole in the M82 galaxy,” is available here.
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Good article! I especially liked the record of the data, because I was curious about what was meant by a black hole being “small”/light in comparison to a massive one (MBH). It turns out, however, that the upper limits to black holes moves far beyond tens of thousands of solar masses. In my reading, many of the MBH at the centers of galaxies weigh in at 10^8-10^9 times the mass of the Sun.