Ultraluminous X-ray sources (ULX) are extragalactic sources with luminosities that exceed any known stellar process. Their origin, though, remains unclear. They are most likely X-ray binaries or binary stars where one object is a star and the second is a compact object, e.g. neutron star or black hole. Their extreme luminosities originate from accretion onto the compact object that may also have strong magnetic fields.
In an attempt to identify the nature of the compact object in the case of two ULXs, astrophysicists analyzed their X-ray spectra taken with the XMM-Newton and Chandra space telescopes. The technique is known as high-resolution spectroscopy. This technique has been used in the past but with lower resolution spectra. The analysis revealed an extremely fast outflow from the system as material from the star accretes onto the compact object. The extreme velocity of this outflow indicates very high energies that imply the compact object is a black hole, but a precise measurement of its mass (i.e. whether it is a supermassive black hole or a stellar mass black hole) was not possible with the available data. Scientists also found that there is a collisional battle between the accretion outflow and a wind from the donating star.
More observational data with better resolution will allow astrophysicists in the future to solve the mystery of the nature of Ultraluminous X-ray Sources.
Publication: Pinto et al. 2016