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 |$\Gamma = 3.17^{+0.22}_{-0.20}$|⁠, but is better fitted with a kTin = 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 (kTin ∼ 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.