A helium nova in the Large Magellanic Cloud -- the faint supersoft X-ray source [HP99]159

TL;DR

This paper proposes that the long-lasting faint supersoft X-ray source [HP99]159 in the LMC is a helium nova in its decaying phase, representing the first such identification in the LMC and the second overall.

Contribution

It introduces a helium nova model for [HP99]159, providing a new interpretation of its nature and linking it to potential Type Ia supernova progenitors.

Findings

Consistent with a 1.2 M_sun white dwarf model.

First helium nova in the LMC.

Supports the helium nova interpretation for long-lived SSSs.

Abstract

We propose a helium nova model for the Large Magellanic Cloud (LMC) supersoft X-ray source (SSS) [HP99]159. This object has long been detected as a faint and persistent SSS for about 30 years, and recently been interpreted to be a source of steady helium-shell burning, because no hydrogen lines are observed. We find that the object can also be interpreted as in a decaying phase of a helium nova. The helium nova is slowly decaying toward the quiescent phase, during which the observed temperature, luminosity, and SSS lifetime ($\gtrsim 30$ years) are consistent with a massive white dwarf model of $\sim$ 1.2 $M_\odot$. If it is the case, this is the second discovery of a helium nova outburst after V445 Pup in our Galaxy and also the first identified helium nova in the LMC. We also discuss the nature of the companion helium star in relation to Type Ia supernova progenitors.