X-ray spectral and optical properties of a ULX in NGC 4258 (M106)

Abstract

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 {\it XMM-Newton}, {\it Chandra}, {\it Swift}, and {\it Hubble Space Telescope} ({\it HST}) observations. The source has a peak luminosity of $L_{\mathrm{X}}\sim 2\times 10^{39}$ erg s${}^{-1}$ in the {\it XMM-Newton} observation of 2004 June. Consideration of the hardness ratios and spectral model parameters shows that the source seems to exhibit possible spectral variations throughout the X-ray observations. In the images from the {\it HST}/Advanced Camera for Surveys (ACS), three optical sources have been identified as counterpart candidates within the 1$\sigma$ error radius of 0$\arcsec$.3. The brightest 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 $-$5.3 mag. The possible spectral types of the candidates from brightest to dimmest were determined as B6$-$A5, B0$-$A7, and B2$-$A3, respectively. The counterparts of the X-ray source possibly belong to a young star cluster. 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 $10-15M_{\sun}$ indicating that the compact source is most likely a stellar-mass black hole.

10 pages, 8 figures, accepted for publication in ApJ

References in corpus (15)

• X-ray Properties of Black-Hole Binaries
• An Ultraluminous X-ray Source Powered by An Accreting Neutron Star
• Super-critically accreting stellar-mass black holes as ultraluminous X-ray sources
• XMM-Newton observations of the brightest Ultraluminous X-ray sources
• A new Cepheid distance to the maser-host galaxy NGC 4258 and its implications for the Hubble Constant
• A mass of less than 15 solar masses for the black hole in an ultraluminous X-ray source
• Iron line and diffuse hard X-ray emission from the starburst galaxy M82
• Spectral State Transitions of the Ultraluminous X-ray Sources X-1 and X-2 in NGC 1313
• Optical emission from massive donors in ULX binary systems
• Optical Properties of the Ultraluminous X-ray Source Holmberg IX X-1 and its Stellar Environment
• Optical Spectroscopy of the environment of a ULX in NGC 7331
• The slim-disk state of the ultraluminous X-ray source in M83
• Transition to the disk dominant state of a new ultraluminous X-ray source in M82
• Spectral State Transitions of the Ultraluminous X-ray Source IC 342 X-1
• A search for periodicities from a ULX in the LINER galaxy NGC 4736

Cited by in corpus (1)

• Optical Counterparts of ULXs and Their Host Environments in NGC 4490/4485