A mass of less than 15 solar masses for the black hole in an ultraluminous X-ray source

Understand

Most ultraluminous X-ray sources (ULXs) display a typical set of properties not seen in Galactic stellar-mass black holes (BHs): higher luminosity Lx > 3 10^39 erg/s, unusually soft X-ray components (kT < 0.3 keV) and a characteristic downturn in their spectra above ~ 5 keV.

• Such puzzling properties have been interpreted either as evidence of intermediate-mass BHs, or as emission from stellar-mass BHs accreting above their Eddington limit, analogous to some Galactic BHs at peak luminosity.
• Recently, a very soft X-ray spectrum has been observed in a rare and transient stellar-mass BH.
• Here we show that the X-ray source P13 in the galaxy NGC\,7793 is in a ~ 64 day period binary and exhibits all three canonical properties of ULXs.