X-ray outbursts from a new transient in NGC 55

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 $\mathrm{\Gamma }=3.17_{-0.20}^{+0.22}$, 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 ($\mathrm{\Gamma }\sim 2.5$–2.9) or a disk blackbody ($kT_{in}\sim 0.8$–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.