X-ray Spectral and Optical Properties of a ULX in NGC 4258 (M106)

We study the X-ray and optical properties of the ultraluminous X-ray source (ULX) X-6 in the nearby galaxy NGC 4258 (M106) based on the archival XMM-Newton, Chandra, Swift, and Hubble Space Telescope (HST) observations. The source has a peak luminosity of L _X ∼ 2 × 10³⁹ erg s^-1 in the XMM-Newton observation of 2004 June. Consideration of the hardness ratios and the spectral model parameters shows that the source seems to exhibit possible spectral variations throughout the X-ray observations. In the images from the HST/Advanced Camera for Surveys, three optical sources have been identified as counterpart candidates within the 1σ error radius of 0.″3. The brightest one has an absolute magnitude of M _V ≈ -7.0 and shows extended structure. The remaining two sources have absolute magnitudes of M _V ≈ -5.8 and -5.3. The possible spectral types of the candidates from brightest to dimmest were determined as B6-A5, B0-A7, and B2-A3. The counterparts of the X-ray source possibly belong to a young star cluster. Neither the standard disk model nor the slim disk model provides firm evidence to determine the spectral characteristics of ULX X-6. We argue that the mass of the compact object lies in the range 10-15 M _⊙, indicating that the compact source is most likely a stellar-mass black hole.