Optical Counterparts of an Ultraluminous X-Ray Source X-1 in NGC 2500

Abstract

We present the results of a search for optical counterparts of ultraluminous X-ray source (ULX) X-1 in the nearby galaxy NGC 2500 by using archival images taken with the Hubble Space Telescope ({\it HST}) Wide Field Camera (WFC3)/UVIS. Four optical sources have been identified as possible counterparts within the 2$\sigma$ error radius of 0.3 arcsec in the images. However, only two of them were investigated as candidates for counterparts due to their point-like features and their identification in various filters. These two faint candidates have absolute magnitudes of $M_{\rm V}$ $\approx$ -3.4 and -3.7. Also possible spectral classes of them were determined as B type main sequence stars. The ages and the masses of the candidates from Color Magnitude Diagram (CMD) were estimated as 45 Myr and 7 $M_{\rm \odot}$, respectively. The spectral energy distributions (SEDs) of two candidates were modeled by a power-law spectrum with a photon index ($\alpha$) $\sim$1.5. The spectra with such slopes could be interpreted as an evidence of reprocessing of the X-rays in the outer part of the disk that generates optical emission.