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-Newton and 2004 Chandra observations, but detected in a 2010 XMM-Newton observation at a significance level of 9σ in the 0.3-8 keV energy band. The XMM-Newton spectrum is consistent with a power law with photon index Γ = 3.17^{+0.22}_{-0.20}, but is better fitted with a kT_in = 0.70 ± 0.06 keV disc blackbody. The luminosity was ∼10³⁸ 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 (X-ray Telescope) observations revealed that the source exhibited recurrent outbursts with period about a month. The XRT spectra can be described by a power law (Γ ∼ 2.5-2.9) or a disc blackbody (kT_in ∼ 0.8-1.0 keV), and the luminosity was in a range of 10³⁸-10³⁹ 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.