A half-century ago, black holes were thought of as little more than bizarre, hypothetical ideas arising from some of Einstein's equations taken to their extreme. Today, black holes are accepted by astronomers as a basic component of the Universe. Under the "standard model" of black holes, they come in two classes: the stellar and the supermassive.
And now, thanks to the efforts of two teams of scientists working independently, there may be a new, third class: the intermediate mass. Although the case for their existence dates back several years, the new findings provide much stronger, although not yet conclusive, proof that such a class of black holes really exists. The only problem is, the standard model just can't account for such a type of black hole.
The stellar class includes what is now viewed as the garden-variety black hole, thought to be strewn throughout most, if not all, galaxies. Such a black hole forms when the especially heavy core of a dying star collapses onto itself under its own crushing gravity. These black holes have masses anywhere from three to ten times the mass of our Sun -- but compressed down to a singular point. (Because no information can escape a black hole beyond its event horizon, the exact nature of this "singularity" is unknown.)
Supermassive black holes (SMBHs) exist at the very center of many galaxies, including our own Milky Way Galaxy. SMBHs usually weigh in on the order of millions or billions of times that of our Sun. How such monsters have developed isn't completely clear but astronomers now believe SMBHs and their host galaxies probably "co-evolved" together over time.
But what about intermediate mass black holes?Defined loosely as having a mass anywhere from 100 to 1000 times that of the Sun, intermediate mass black holes (IMBHs) theoretically shouldn't exist because there is no really good way to create them. No single star could ever be large enough to create such a heavy black hole. And while evidence suggests that colliding galaxies result in coalescing SMBHs, random stellar black hole mergers are probably too rare to produce IMBHs.
Using a pair of X-ray telescopes, Dr. Tod Strohmayer and Dr. Richard Mushotzky at the NASA Goddard Space Flight Center in Greenbelt, Md., and a team led by Dr. Jon Miller at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., determined the masses of three suspected intermediate black holes.
Miller's team measured the temperature of the superheated gas believed to make up the swirling accretion disk of material being pulled in by two of the black holes. Then by utilizing the known inverse relationship between the mass of a black hole and the temperature and velocity of the gas at different locations within the disk, they estimated the masses of these two black holes between 200 and 500 solar masses, making them IMBHs.
"Evidence is mounting that these elusive intermediate mass black holes may really exist," Miller said. "The mystery, really, is how they can exist."
The Strohmayer and Mushotzky team measured variations in the energy being emitted by the accretion disk surrounding the third black hole (located in a separate galaxy). In addition to visible light, accretion disks are so hot they emit lots of X rays. The two wanted to know if this black hole appeared so "bright" in X rays because its orientation to the Earth is such that all the X rays are "beamed" in our direction. The effect is similar to a shining a flashlight in someone's eyes, except in X rays. This "brightness" would make the stellar-mass black hole seem much heavier than it actually is. They concluded it is highly unlikely all the X rays from the accretion disk would be beamed toward the Earth.
The two also examined the X-ray emissions from iron atoms in the accretion disk. This is because X-ray emission from iron atoms look different if the atoms are moving extremely fast. Fast moving atoms occur in the accretion disk due to the pull of the black hole. Specifically, the emission lines appear wider (broader). The presence of the "broad iron line" is a telltale sign of a black hole.
With both measurements, the Goddard team estimated the black hole's mass to be at least 50 times greater than a stellar one, making it an intermediate mass candidate.
Both studies necessarily relied on indirect evidence. Future studies may provide more evidence or answers. And if it turns out IMBHs do in fact exist, the standard model of black hole formation will have to be rewritten to include them.