Searching for Outflows in Spectra of Ultraluminous X-ray Sources

Ultraluminous X-ray sources are non-nuclear point sources exceeding the isotropic Eddington luminosity of a 10 Solar mass black hole. They could be powered by either sub-Eddington intermediate mass black holes, or lighter objects (stellar-mass black holes or neutron stars) accreting at super-Eddington ratios. The second variant predicts existence of powerful outflows of highly ionised gas at relativistic velocities. So far, they have been found in 2 systems: NGC 1313 X-1 and NGC 5408 X-1. We attempt to find signatures of such outflows in 2 different sources using the RGS spectrometer onboard XMM-Newton. Holmberg IX X-1, a very hard and variable ULX alike NGC 1313 X-1, and Holmberg II X-1, much softer and similar to NGC 5408 X-1. Using systematic Gaussian line search we find evidence for absorption and emission features which are unlikely to be caused by background or random fluctuations. Rigorous search for an ionised absorber at relativistic velocities in Holmberg IX X-1 spectrum finds possible detection of wind at 0.25c, similar to what has been detected in NGC 1313 X-1 and predicted by super-Eddington accretion. For Holmberg II X-1, we find results similar to NGC 5408 X-1 in agreement with their broadband spectral shape.