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 $10^{36}-2\times10^{38}$ 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 $\gtrsim 2\times10^{60}$ cm$^{-3}$ corresponds to a wind mass-loss at rates $\gtrsim 2\times 10^{-6}$ $M_{\odot}\,{\rm yr}^{-1}$. 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.