http://arxiv.org/abs/1607.02881
We study the X-ray and optical properties of Ultraluminous X-ray source (ULX) X-6 in the nearby galaxy NGC 4258 (M106) based on the archival XMM-Newton, Chandra, Swift, and HST observations. The source has a peak luminosity of $L_{\mathrm{X}} \sim 2 \times 10^{39}$ erg s$^{-1}$ in the XMM-Newton observation of 2004 June. Throughout the X-ray observations, the source seems to exhibit possible spectral variations by considering the hardness ratios and the spectral model parameters. In the HST/ACS images, three optical sources have been identified as counterpart candidates within the 1$\sigma$ error radius of 0.3 arcsec. The brighter one has an absolute magnitude of M$_{V} \approx$ $-$7.0 and shows extended structure. The remaining two sources have absolute magnitudes of M$_{V} \approx$ $-$5.8 and $-$5.3 mag. The possible spectral types of the candidates from brighter to dimmer were determined as B6$-$A5, B0$-$A7 and B2$-$A3, respectively. The counterparts of the X-ray source possibly belong to a young star cluster. Both the standard disk model and the slim disk model do not provide a firm evidence to determine the spectral characteristics of ULX X-6. We argue that the mass for the compact object lies in the range 10-15M$_{\odot}$ indicating that the compact source is most likely a stellar-mass black hole.
H. Avdan, S. Avdan, A. Akyuz, et. al.
Tue, 12 Jul 16
40/71
Comments: 10 pages, 8 figures, accepted for publication in ApJ
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