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 9 \sigma in the 0.3–8 keV energy band .
The XMM-Newton spectrum is consistent with a power law with photon index \Gamma = 3.17 ^ { +0.22 } _ { -0.20 } , but is better fit with a kT _ { 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 ( 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 .