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 a steady hydrogen burning on its surface generating bolometric luminosities of 10 36 − 2 × 10 38 erg s −1 . 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 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 of a few times 10 38 − 10 39 erg s −1 , radiating at blackbody temperatures of ≈ 3 × 10 5 K, and indicate nebular continuum, whose emission measure of 2 × 10 60 cm −3 corresponds to a wind mass-loss at rates 2 × 10 −6 M ⊙ yr −1 . Such the 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 to understand their proper stage in stellar evolution.