Ultra-luminous x-ray sources
(PhysOrg.com) -- An ultra-luminous X-ray source (ULX) emits more radiation in the X-rays than do a million suns at all wavelengths. ULXs are rare: Most galaxies, including our own Milky Way, have none, and galaxies that do host a ULX usually have only one. ULXs are also mysterious: If they were stars with about the mass of the sun, their huge luminosities should tear them apart. Most astronomers today think that ULXs are black holes of more than about ten solar-masses that are accreting material onto a disk, and emitting their X-rays in a beam. When there is beaming towards us, the total luminosity is much less than we would infer from our measurements than if the emission were in all directions. This explanation helps understand the physics of ULXs, but their origins are still obscure.