K. L. Li, Albert Kong, P. A. Charles, Ting-Ni Lu, E. S. Bartlett, M. J. Coe, V. McBride, A. Rajoelimanana, Andrzej Udalski
We present X-ray, optical and radio observations of the 2011 transient X-ray source MAXI J0158-744 that reveal some remarkable properties. Detected as a new X-ray transient by MAXI/GSC on 11 Nov 2011, we followed the subsequent exponential decline of the X-ray lightcurve with Swift observations over the following month. All of the Swift spectra exhibit low temperatures (~100eV) indicating that MAXI J0158-744 is a new Supersoft Source (SSS). The Swift X-ray spectra near maximum show features around 0.8keV that we interpret as possible absorption from OVIII and emission from O, Fe and Ne lines. We obtained SAAO and ESO optical spectra of the counterpart early in the outburst and several weeks later. The later spectrum reveals absorption features that indicate a B1/2IIIe spectral type and all spectral features are at velocities consistent with the Wing of the SMC. At this distance, it is a luminous SSS (>10^37 erg/s) but whose brief peak luminosity of >10^39 erg/s in the MAXI/GSC 2-4 keV band makes it the brightest SSS yet seen at "hard" X-ray energies. We therefore propose that MAXI J0158-744 is a Be-WD binaries, and the first example to possibly enter ULX territory. Because the companion to the WD is in this case an early B star, we propose that the brief hard X-ray flash is a result of the shock-heated interaction of the ejected nova shell with the B star wind in which the WD is embedded. This makes MAXI J0158-744 only the third Be/WD system in the Magellanic Clouds, but it is by far the most luminous. While the SMC has an extensive population of Be X-ray pulsars, the detection of their white dwarf cousins is hampered by their greatly reduced optical outburst amplitude and lack of continuous soft X-ray monitoring of these regions. However, the properties of MAXI J0158-744 now gives weight to previous suggestions that SSS in nearby galaxies are associated with early-type stellar systems.
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http://arxiv.org/abs/1207.5023
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