X-ray outbursts from a new transient in NGC 55

V. Jithesh, Zhongxiang Wang

Published 2014-11-24Version 1

We report the outbursts from a newly discovered X-ray transient in the Magellanic-type, SB(s)m galaxy NGC 55. The transient source, XMMU J001446.81-391123.48, was undetectable in the 2001 XMM-Newton and 2004 Chandra observations, but detected in a 2010 XMM-Newton observation at a significance level of 8 sigma in the 0.3-8 keV energy band. The XMM-Newton spectrum is consistent with a power law with photon index of $\Gamma=3.04^{+0.21}_{-0.19}$ or a disk blackbody with $kT_{in} = 0.73\pm0.06$ keV. The luminosity was ~$10^{38}$ erg/s, and the source displayed strong short-term X-ray variability. These results, combined with the hardness ratios of its emission, strongly suggest an X-ray binary nature for the source. The follow-up studies with Swift XRT observations revealed that the source exhibited recurrent outbursts with period about a month. In the faint state, the XRT spectrum is described by a power law ($\Gamma$ ~ 2.5), and during the outburst, the source reached its peak luminosity ~ 2x$10^{39}$ erg/s without evidence for any changes of the photon index, which may be an indication of steep power law state outburst. Based on the X-ray spectral and temporal properties, we conclude that XMMU J001446.81-391123.48 is a new transient X-ray binary in NGC 55, which possibly contains a black hole primary.