The ULX source X-3 in NGC 4258: a search for its X-ray and optical properties

We present the results of a search for the nature of ultraluminous X-ray source (ULX) X-3 in the nearby galaxy NGC 4258. We use archival data from XMM-Newton, Chandra, NuSTAR, and Hubble Space Telescope (HST) observations. Total X-ray data were analysed to find that the model parameters of the system are indicative of a stellar-mass black hole, ∼10 M_⊙, as the central compact object. Furthermore, analyses of the optical data from HST reveal two optical candidates at 90 per cent confidence level within an error radius of 0.28 arcsec. Assuming the optical emission is dominated by the donor star, both of these candidates are found to have spectral types that lie between B3 and F1 with absolute magnitudes of M_V ≈ -6.4. Moreover, the age and mass estimates for the candidates are found to be 10 and 18 Myr and 13 and 20 M_⊙, respectively.