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