New Ultraluminous Supersoft Source in the Small Magellanic Cloud: MAXI J0158-744
Abstract
We present a time-resolved analysis of an ultraluminous supersoft source (SSS) MAXI J0158-744 using 18 consecutive follow-up Swift ToO observations. MAXI J0158-744 is an ultraluminous soft X-ray outburst with a peak luminosity up to 4.3 10^39 erg/s in the energy range of 0.2-2.0 keV located in the Magellanic Bridge region, detected by MAXI/GSC on 11 November 2011. Follow-up Swift observations confirmed that the X-ray emissions are ultra-soft, which could be well fitted by blackbody models of temperatures down to 52 eV. Since the onset, the X-ray emission decreased exponentially dropping from $10^39 to 10^37 erg/s in 15 days (assuming the Small Magellanic Cloud distance) and fell below the detection limit of Swift after 09 December 2011. The earliest Swift X-ray spectra show K-edge absorption (0.88 keV) and a broad Lya (0.65 keV) from OVIII, which indicate that it could be an oxygen rich system. Swift UVOT also caught the outburst in U band with magnitude 13.07 on 12 November 2011 and it returned to quiescence with magnitude 13.6 two days later. During this quiescence, we performed two ATCA radio observations at frequencies 5.5 GHz and 9 GHz with the baseline ranging from 337 m to 6 km and constrained an upper limit of 45 micro-Jansky at the X-ray position. By comparing the object with the close-binary soft source (CBSS) model, we therefore concluded that MAXI J0158-744 could be a slowly accreting O-rich white dwarf binary, with unstable hydrogen burning on the WD surface. Remarkably, ultraluminous SSSs are very rare high-energy phenomena and this nearest one MAXI J0158-744 provided a very unique opportunity for us to understand the underlying physics of such a system.
- Publication:
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American Astronomical Society Meeting Abstracts #220
- Pub Date:
- May 2012
- Bibcode:
- 2012AAS...22052320L