X-ray spectral and optical properties of an ultraluminous X-ray source in NGC 4258 (M106)

We report the X-ray and optical properties of an ultraluminous X-ray source (ULX) in the nearby galaxy NGC 4258 (M106). The XMM-Newton and Chandra archival observations were used to examine the X-ray spectral properties of the source. Throughout the X-ray observations, we discuss that the source appears to exhibit possible spectral changes by considering the hardness ratios and the spectral model parameters. The luminosity of the source varies a factor of two during the observations and has a peak value of ∼2x10^{39} erg s^{-1}. In the optical band, the source seems to belong to a star cluster. The archival HST images were used to search the optical counterpart of the ULX and three possible candidates were found within the 1σ error radius of 0.3". Also the mass for the compact object is estimated in the range of 10-15 M _{sun} which indicates a stellar-mass black hole.