X-ray Spectral Studies of the Ultra-luminous X-ray Source M81 X-6

We present the X-ray spectral studies of the ultra-luminous X-ray source (ULX) M81 X-6 using the Suzaku and XMM-Newton observations performed during 2001 - 2015. We systematically analysed the observed spectra with different physical models including a multi-temperature blackbody model for a thin, steady state, general relativistic accretion disc around a Kerr black hole. Spectral modelling results suggest that the source exhibits three distinct spectral shapes with different luminosity in these observations. Using the relativistic accretion disc model, we attempt to constrain the mass of the black hole, mass accretion rate and spin of the system. Our analysis suggests a maximally rotating black hole for M81 X-6 with a black hole mass of < 100 solar mass, which is broadly consistent with the scenario that this ULX is a stellar-mass black hole emitting at Eddington or super-Eddington luminosity.