The mass ranges of normal stars, white dwarf stars, and neutron stars are pretty well constrained by modern astrophysics. Not so the mass range of black holes, those mysterious singularites that lie both within and outside of our Universe. Black holes which form from stars start out with masses of a few times the mass of our Sun. There are also supermassive black holes, like Sgr A*, the one that lurks at the center of our Milky Way. The relation of these "stellar mass" and "supermassive" black holes is a matter of conjecture: do stellar mass black holes evolve into supermassive black holes by swallowing enormous quantities of stars and gas? Or are they formed by completely separate mechanisms? Astronomers have been searching for some time for "intermediate-mass" black holes with masses of hundreds or thousands of solar massers, much larger than stellar mass black holes but much smaller than the supermassive variety. Perhaps such intermediate mass black holes represent a transitional stage between "stellar" and "super" mass. These intermediate-mass black holes are hard to find, however. Maybe the best evidence of the existence of these objects are the so-called "ultra-luminous X-ray sources". The sources are believed to be accreting gas (and maybe even whole stars) at a prodigious rate, and as a result have X-ray luminosities that are orders of magnitude higher than typical stellar mass black hole systems, but orders of magnitude lower than accreting supermassive black holes. The image above shows an unusual ULX in the galaxy M83, detected by the Chandra X-ray Observatory. An optical image of M83 is shown on the left, while a composite X-ray/optical image is shown on the right. The X-ray emission is shown on the right image in pink, and the source marked by the arrow is a peculiar ULX which brightened by over a factor of 3000 in about 10 years. Why it brightened is a mystery. There are also substantial puzzles about how this object could have formed. It may be an ancient black hole left over from he days when M83 was very young. Some researchers think that this strange source could even be a stellar mass black hole, despite its extreme X-ray behavior. It will be of intense interest to follow future developments of this strange object.
CXC: M83: Remarkable Outburst from Old Black Hole
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