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

TL;DR

This study models the full spectral energy distribution of luminous supersoft X-ray sources in the Magellanic Clouds, revealing their potential as post-nova systems with high mass-loss rates and sustained accretion.

Contribution

First comprehensive multiwavelength SED modeling of the brightest SSSs in the Magellanic Clouds, linking their emission to nebular outflows and accretion processes.

Findings

Bright SSSs have luminosities of 10^{38}-10^{39} erg/s and temperatures around 3x10^5 K.

SED modeling indicates nebular emission with high emission measures and wind mass-loss rates.

Bright SSSs may be unidentified post-novae with sustained high accretion rates.

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.