Investigation of X-ray and optical properties of ultraluminous x-ray sources in NGC 4861, NGC 4449 galaxies

Ultraluminous X-ray Sources, (ULXs) are binary star systems located outside the nuclear region of galaxies and have much higher luminosities (LX> 1039 ergs-1) than Galactic X-ray binary systems. In this study, possible optical counterparts of ULX-1, ULX-2 sources in the NGC 4861 galaxy and ULX-1 source in the NGC 4449 galaxy were first identified using Hubble Space Telescope (HST) archival data. Only for NGC 4861 ULX-2, two optical candidates with absolute magnitudes, Mv, -6 and -5 were defined within an error radius of 0.28" (90% confidence level). Spectral types and ages of both candidates were determined as F0I and ~ 31Myrs, respectively. However, in NGC 4449, three optical candidates were identified for the ULX-1 source within an error radius of 0.20 "(90% confidence level). MV values of the optical candidates lie between −5 and −3. Their spectral types and ages are between B0III- B3V and 13Myr- 63Myr, respectively. Considering the location of ULX-1 (NGC 4449) to the nearby cluster, it was investigated whether there is an association with the cluster. In addition, for comparison with previous studies, new (unused) archival data of Chandra and XMM-Newton were analyzed and X-ray spectral properties of the sources were examined. No significant changes were observed in the X-ray characteristics of the sources.