Investigating the Nature of Ultraluminous X-ray Sources through X-ray and Optical data

Ultraluminous X-ray sources (ULXs) are point-like objects that emit at luminosities intermediate between those of accreting Stellar Mass Black Holes and Super Massive Black Holes. If the emission is isotropic in these binary systems, then the central object is an Intermediate Mass Black Hole (IMBH). Conversely, some ULX properties can be explained assuming that their emission is beamed and the central BH has a stellar mass. Discriminating between a binary system hosting a solar mass BH and one with an IMBH is an extremely challenging task. I studied two bright ULXs: NGC 1313 X-2 and M82 X-1. At present NGC 1313 X-2 is one of the best studied ULXs. Its X-ray spectrum (from XMM-Newton data) is well fitted with models of the type ``soft thermal component + hard tail''. The temperature of the soft thermal component, if interpreted as emission from an accretion disk, implies a BH mass of at least 60 solar masses. Crucial information have been obtained also from the study of the optical counterpart. We first identified the optical counterpart of NGC 1313 X-2 on an ESO 3.6m image. VLT and HST images allowed us to resolve the counterpart in two distinct objects. Our model compared with the available optical and X-ray data indicates that NGC 1313 X-2 is likely to be an IMBH X-ray binary with a relatively massive main sequence donor which fills its Roche lobe. M82 X-1 is one of the most luminous ULXs and is the first source in which a Quasi-Periodic Oscillation (QPO) was detected. Our analysis of XMM-Newton and RXTE observations gives useful constraints for the mass of the BH hosted in this ULX. The properties of the observed QPOs are reminiscent of that of the low-frequency, type-C QPOs observed in BHCs. Scaling the frequency inversely to the BH mass, the observed QPO frequency range would yield the BH mass anywhere in the interval 10 to 1000 solar masses. Some arguments have been proposed to reduce the uncertainty on the determination of the mass of the compact object in M82 X-1, using in particular the correlation between the spectral and timing properties observed in BHCs. It is tempting to conclude that a likely value for the mass of M82 X-1 may be a few hundreds solar masses.