Some superbright x-ray sources may be unusual pulsars

Ars Technica : Since their discovery in the 1970s, ultraluminous x-ray sources (ULXs) continue to puzzle astronomers. At first they were thought to be stellar-sized black holes, but their luminosity exceeded a theoretical limit based on their masses. Recent observations have suggested that they are intermediate-sized black holes, with masses on the order of 100 to 100 000 times that of the Sun. But an unexpected finding about a known ULX in the M82 galaxy may complicate that possibility. Deepto Chakrabarty of MIT and his colleagues were using NASA’s Nuclear Spectroscopic Telescope Array to watch the galaxy for a new supernova when they noticed that the brightness of ULX X-2 was pulsing, something that black holes don’t do. Using the Chandra X-Ray Telescope, the researchers confirmed that it was in fact X-2 that was pulsing. That makes it possible that X-2 and other ULXs are pulsars—bright, rapidly rotating neutron stars—and not black holes. However, X-2 is significantly brighter than any other known pulsars. The same limit to black hole luminosity applies to pulsars, and X-2 exceeded the limit by a factor of 100. Chakrabarty recalculated X-2’s mass using the version of the brightness limit that accounts for a pulsar’s magnetic field and found that while the ULX’s mass had increased, it was still two to three times brighter than it should be. To further confuse the situation, X-2 has a sibling, ULX X-1, that shows no sign of being a pulsar.