Mystery of luminous supersoft X-ray sources in Large Magellanic Cloud and Small Magellanic Cloud
Authors:
Augustin Skopal
Image & caption:
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Image caption::
Top (a): Our model of the spectral energy distribution (SED) from the super-soft X-rays to near-infrared (heavy black line) for the brightest SSS in LMC, RXJ0513.9-6951. The model fits well with the observed fluxes (in magenta) and corresponds to the luminosity of 2E+39 erg/s. Open squares are the observed X-ray fluxes. Bottom (b): Comparison of our SED model with the currently considered model (heavy gray line) which corresponds to the Eddington luminosity, 2E+38 erg/s, but does not fit the measured data. This fact shows the necessity of SED modeling in a wide range of the global spectrum.
Description:
In the late 1980s and early 1990s, the X-ray satellites Einstein and ROSAT discovered very luminous super-soft X-ray sources (SSS) in the Large and Small Magellanic Clouds (LMC and SMC). Since 1992, they have been understood as close binaries in which a massive white dwarf accreting matter from its companion generates a luminosity of 1E+36 to 2E+38 erg/s. Applying our own method of modeling the spectrum from super-soft X-rays to near-infrared for the brightest SSS in the LMC and SMC revealed a surprising result: The luminosity of the brightest SSS is several times 1E+38 to 1E+39 erg/s, which is an order of magnitude more than the so-called Eddington limit at which radiation pressure balances gravity. That is, it is much higher than the upper limit of the luminosity of normal stars. In the work, I propose a new view of the nature of the brightest SSS, as unidentified optical novae, which are in the phase of a super-soft X-ray source, which is maintained at high luminosity and for a long time by a rapid resumption of accretion. Further investigation of these mysterious objects in a wide range of the electromagnetic spectrum will help us understand their proper stage in stellar evolution.
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