Discovery of a Highly Variable ULX Within NGC 4736
Abstract
We report our discovery of a new ultraluminous X-ray source within the Seyfert galaxy NGC 4736, which shows many intriguing properties. The source has been observed by ROSAT, Chandra, and XMM-Newton on several occasions, with a flux variation of at least a factor of 26 and a peak absorbed luminosity of 2.1x10^{39} erg/s in the 0.3-10 keV band. The source is better fitted with a multicolor disk in the high state, but a hard powerlaw is favored in the low state, indicating the observation of a possible spectral state transition that is similar to that seen in Galactic black-hole X-ray binaries. In the brightest observation, which was made by XMM-Newton, we also find some evidence of dipping. In a 14 hr low-state observation made by Chandra, the light curve seems to show a ~7 hr periodic modulation over two cycles. Thus, the source is probably a highly variable, dipping and periodic black-hole X-ray binary with the black-hole mass about a few tens of solar mass. We discuss the importance of this source on helping us understand the nature of the ultraluminous X-ray sources.
- Publication:
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AAS/High Energy Astrophysics Division #13
- Pub Date:
- April 2013
- Bibcode:
- 2013HEAD...1312652L