Unusual bright x-ray sources may have an explanation

Ars Technica : A class of astronomical objects known as ultraluminous x-ray sources (ULXs) is thought to be made up of a mix of black holes and neutron stars, but what causes them to radiate so brightly in the x-ray band is unknown. An examination of four ULXs that are relatively close and easy to observe in the optical band may provide some clues. All four share spectral characteristics with either supergiants and luminous blue variable (LBV) stars, and because the ULXs are all binary systems, the researchers interpreted the spectra as the result of the outflow of hot material. The outflow material, which is significantly hotter than either supergiants or LBVs, could be produced by the partner star’s wind or from a black hole’s accretion disk. All four ULXs also share spectral characteristics with SS 433, the galaxy’s only known supercritical accretor, which appears to be attracting material faster than theoretically possible. The researchers suggest that the ULXs they examined could also be supercritical accretors, just not at the same scale as SS 433.