Searching for Outflows in Ultraluminous X-ray Sources Through High-Resolution X-ray Spectroscopy

Ultraluminous pulsars are a definite proof that persistent super-Eddington accretion occurs in nature. They support the scenario according to which most Ultraluminous X-ray Sources are super-Eddington stellar mass accretors rather than intermediate mass black holes. An important prediction of theories of supercritical accretion is the existence of powerful outflows of moderately ionized gas at relativistic speeds (0.2-0.3c). In practice, the spectral resolution of X-ray gratings such as RGS onboard XMM-Newton is required to resolve their observational signatures in ULXs. Using RGS, the outflows have been discovered in the spectra of 3 ULXs (none of which are currently known to be pulsars). We studied a sample of 10 ULXs and found one which shows evidence (3σ) for a relativistic jet with a projected speed of 0.34c, another feature widely predicted by the models of super-Eddington accretion. The rest do not have high enough data quality for a significant detection but most show signatures similar to those of ULXs with known outflows. A majority also exhibits narrow emission lines which could originate in photoionized, or shocked gas (as the potential outflow hits the surrounding medium). Most recently, the 4th ultraluminous pulsar was discovered in NGC 300. I will present the results of a rigorous, in-depth search for a relativistic wind in the spectrum of this object using all available X-ray data.