Multiwavelength modeling the SED of Luminous Supersoft X-ray Sources in Large Magellanic Cloud and Small Magellanic Cloud

Abstract

Classical supersoft X-ray sources (SSSs) are understood as close binary systems in which a massive white dwarf (WD) accretes from its companion at rates sustaining steady hydrogen burning on its surface generating bolometric luminosities of $10^{36}-2\times 10^{38}$ erg/s. Here, we perform for the first time the global supersoft X-rays to near-infrared (NIR) spectral energy distribution (SED) for the brightest SSSs in LMC and SMC. We test a model in which the ultraviolet--NIR is dominated by the emission from a compact (unresolved) circumstellar nebula represented by the ionized gas out-flowing from the SSS. The SED models correspond to luminosities of SSSs a few times $10^{38}-10^{39}$ erg/s, radiating at blackbody temperatures of $\approx 3\times 10^5$ K, and indicate nebular continuum, whose emission measure of $\gtrsim 2\times 10^{60}$ cm${}^{-3}$ corresponds to a wind mass-loss at rates $\gtrsim 2\times 10^{-6}$ $M_{\odot }{\mathrm{yr}}^{-1}$. Such extreme parameters suggest that the brightest SSSs could be unidentified optical novae in a post-nova SSS state sustained at a high long-lasting luminosity by resumed accretion, possibly at super-Eddington rates. New observations and theoretical multiwavelength modeling of the global SED of SSSs are needed to reliably determine their parameters, and thus understand their proper stage in stellar evolution.

arXiv comment26 pages, 9 figures, 4 tables, accepted for AJ

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