X-ray outbursts from a new transient in NGC 55

Abstract

We report the outbursts from a newly discovered X-ray transient in the Magellanictype, 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 fit with a kT in = 0.70 ± 0.06 keV disk blackbody. The luminosity was ∼ 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 (Γ ∼ 2.5-2.9) or a disk blackbody (kT in ∼ 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.

A transient ultraluminous X-ray source in NGC 55

“…3, left) produced with the online tool (Evans et al 2007) 3 . The Swift/XRT monitoring confirms the strong long-term variability and underlines the presence of multiple high-flux states as previously suggested by Jithesh & Wang (2015). In Fig.…”

“…We requested new observations to follow up ULX-1 and search for any new transients or strong variability in the other bright X-ray binaries in the NGC 55 galaxy. Among them, XMMU J001446.81-391123 in the past showed the most remarkable variability (Jithesh & Wang 2015. Considering the transient nature of some ULXs, the newly discovered ones are not always necessarily "new" sources.…”

“…Therefore, at higher 𝑀 the source would get even softer (Middleton et al 2015). For more information on the lower states of ULX-2, see Jithesh & Wang (2015) who suggests that the source is likely a stellar-mass BH or NS XRB. A wind seems indeed to appear in the brightest (ULX) state as shown by the features observed around 1 keV (see Fig.…”