Searching for optical counterparts of ultraluminous X-ray sources in nearby galaxies: NGC 5474, NGC 3627, NGC 4258 and NGC 4490/4485

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 > 1039 erg s-1). Altough it is known that ULX, which is determined from Chandra and/or XMM-Newton X-ray telescope observations is a binary star system, its component stars and their emission mechanism are not yet well understood. Studying the optical counterparts of ULXs are important to understand emission mechanisms. In this thesis, the optical counterparts of ULXs in the nearby (d < 10 Mpc) NGC 5474, NGC 3627, NGC 4258 and interacting NGC 4490/4485 galaxies were detected for the first time and studied in detail. Hubble Space Telescope (HST) archival data, SAO BTA-6 m and TUG RTT-150cm telescopes were used for photometric and spectroscopic analyses. The mass, age and spectral type of the secondary stars were calculated by using the HST mutli-band magnitutes of the possible optical counterparts. Also possible associations between ULXs and star cluster or star groups in their environments were discussed.