Discovery of a ~30-Year-Duration Post-Nova Pulsating Supersoft Source in the Large Magellanic Cloud

TL;DR

This paper reports the discovery and long-term study of a rare, ~30-year-duration post-nova supersoft X-ray source in the Large Magellanic Cloud, revealing a white dwarf undergoing residual nuclear burning after a nova event.

Contribution

It presents the first identification of a long-lived post-nova supersoft source with detailed multi-decade X-ray analysis, confirming theoretical predictions.

Findings

Identified a long-lived (~30 years) post-nova supersoft source in the LMC.

Measured short period oscillations of ~170 seconds in X-ray data.

Confirmed the source's evolution aligns with models of a ~0.7 solar mass white dwarf.

Abstract

Supersoft X-ray sources (SSS) have been identified as white dwarfs accreting from binary companions and undergoing nuclear-burning of the accreted material on their surface. Although expected to be a relatively numerous population from both binary evolution models and their identification as Type Ia supernova progenitor candidates, given the very soft spectrum of SSSs relatively few are known. Here we report on the X-ray and optical properties of 1RXS J050526.3-684628, a previously unidentified accreting nuclear-burning white dwarf located in the Large Magellanic Cloud (LMC). XMM-Newton observations enabled us to study its X-ray spectrum and measure for the first time short period oscillations of ~170 s. By analysing newly obtained X-ray data by eROSITA, together with Swift observations and archival ROSAT data, we have followed its long-term evolution over the last 3 decades. We identify 1RXS J050526.3-684628 as a slowly-evolving post-nova SSS undergoing residual surface nuclear-burning, which finally reached its peak in 2013 and is now declining. Though long expected on theoretical grounds, such long-lived residual-burning objects had not yet been found. By comparison with existing models, we find that the effective temperature and luminosity evolution are consistent with a ~0.7 $M_{\odot}$ carbon-oxygen white dwarf accreting ~10$^{-9}~\rm{M}_{\odot}$/yr. Our results suggest there may be many more undiscovered SSSs and "missed" novae awaiting dedicated deep X-ray searches in the LMC and elsewhere.