X-ray outbursts from a new transient in NGC 55

Abstract

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{} and 2004 \chandra{} observations, but detected in a 2010 \xmm{} observation at a significance level of $9\sigma$ in the 0.3--8 keV energy band. The \xmm{} spectrum is consistent with a power law with photon index $\Gamma =3.17_{-0.20}^{+0.22}$, but is better fit with a $k{T}_{in}=0.70\pm 0.06$\,keV disk blackbody. The luminosity was $\sim 10^{38}$\,erg\,s${}^{-1}$, 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. The XRT spectra can be described by a power law ($\Gamma \sim 2.5$--2.9) or a disk blackbody ($k{T}_{in}\sim 0.8$--1.0\,keV), and the luminosity was in a range of 10${}^{38}$--10${}^{39}$\,erg\,s${}^{-1}$, with no evidence showing any significant changes of the spectral parameters in the observations. 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.

arXiv commentAccepted for publication in MNRAS; Matched with accepted version; 6 Pages, 5 Figures, 2 Tables

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