Investigation of timing and spectral properties of the ultraluminous X-ray sources in NGC 4736 and NGC 4258

Ultraluminous X-ray sources (ULXs) are extragalactic off-nuclear point-like sources with luminosities exceeding the Eddington limit for a stellar mass black hole (Lx > 10^39 erg s^-1). ULXs are considered as a subclass of X-ray binaries undergoing mass accretion and hosting neutron star or black hole as primary star. In this thesis, detailed X-ray spectral and timing analysis of two ULXs in the nearby (d < 10 Mpc) galaxies NGC 4736 and NGC 4258 were studied. The X-ray data of Chandra, XMM-Newton and Swift satellites were used for these analysis. Timing analysis of the ULX (X-2) in NGC 4736 revealed a mHz quasi-periodic oscillation (QPO). Also, the X-ray spectral evolution of the ULX (X-6) in NGC 4258 were examined by using multiple observations in the archives. The analysis revealed that the source exhibits spectral variation. The calculations that were done by considering the spectral model parameters suggest that the compact objects in both sources are probably stellar mass black holes.