The Nature of the Emission Spectrum of NGC 7793 P13: Testing the Supercritical Accretion Disk Wind Model

The optical spectra of ultraluminous X-ray sources (ULXs) show signs of powerful outflows of matter. These outflows are responsible for the formation of a significant portion of optical and ultraviolet emission in ULXs and can either be stellar winds of the donor stars or optically thick outflows (winds) from the surface of supercritical accretion disks. In the latter scenario the outflows are still expected to be similar to stellar winds of massive stars, which allows one to use the same methods for their study based on a comparison of the observed spectra with those simulated within the framework of non-LTE extended atmosphere models. In this paper, we simulate the optical spectrum of the ultraluminous X-ray pulsar NGC 7793 P13, assuming that its emission part is produced in the wind of the supercritical accretion disk. The estimated mass loss rate is about 1.4× 10^-5M_⊙ yr ^-1 . We consider the positive and negative aspects of the model and also discuss the applicability of the concept of supercritical disk winds to NGC 7793 P13 and to another well-known ultraluminous X-ray pulsar, NGC 300 ULX-1.