Black hole mass estimation of ULXs and its dependence on model parameters

In the present work, we have analysed Chandra observational data of 38 galaxies which are obtained from the public data archive of Chandra X-ray Centre. 75 sources with data counts above 200 are considered for the analysis. The spectra of these sources were fitted using the absorbed disk blackbody model. Based on the estimated X-ray luminosity, we have categorized the sources into X-ray binaries (XRBs), ultraluminous X-ray sources (ULXs), extremely luminous X-ray sources (ELXs) and hyperluminous X-ray sources (HLXs). We estimate the black hole (BH) mass of all the point sources detected in the sample galaxies using the disk blackbody model, assuming that the inner disk radius, R_in ∼ 10GM/c². It is found that most of the ULXs and ELXs harbouring stellar mass black holes are in hard states while those harbouring intermediate mass black holes (IMBHs) are in soft states. In case of HLXs, those harbouring IMBHs are found to be in hard states whereas a small population of soft sources are seen harbouring stellar mass black holes thereby accreting above their Eddington luminosity.