Supersoft X-Ray Phases of Recurrent Novae as an Indicator of their White Dwarf Masses

TL;DR

This study analyzes optical and X-ray light curves of recurrent novae to establish a relation between supersoft X-ray phase duration and white dwarf mass, providing a new method for estimating WD masses.

Contribution

It introduces a correlation between SSS phase duration and WD mass, supported by observational data and theoretical models, enhancing understanding of nova outbursts.

Findings

Six novae show a consistent relation: $t_{SSS} \\approx 0.70 t_{off}$.

SSS duration correlates with WD mass for broad X-ray light curve novae.

Theoretical models support SSS duration as an indicator of WD mass.

Abstract

We have examined the optical/X-ray light curves of seven well-observed recurrent novae, V745 Sco, M31N 2008-12a, LMC N 1968, U Sco, RS Oph, LMC N 2009a, T Pyx, and one recurrent nova candidate LMC N 2012a. Six novae out of the eight show a simple relation that the duration of supersoft X-ray source (SSS) phase is 0.70 times the total duration of the outburst ($=$ X-ray turnoff time), i.e., $t_{\rm SSS}=0.70 t_{\rm off}$, the total duration of which ranges from 10 days to 260 days. These six recurrent novae show a broad rectangular X-ray light curve shape, first half a period of which is highly variable in the X-ray count rate. The SSS phase corresponds also to an optical plateau phase that indicates a large accretion disk irradiated by a hydrogen-burning WD. The other two recurrent novae, T Pyx and V745 Sco, show a narrow triangular shape of X-ray light curve without an optical plateau phase. Their relations between $t_{\rm SSS}$ and $t_{\rm off}$ are rather different from the above six recurrent novae. We also present theoretical SSS durations for recurrent novae with various WD masses and stellar metallicities ($Z=$0.004, 0.01, 0.02, and 0.05) and compare with observed durations of these recurrent novae. We show that the SSS duration is a good indicator of the WD mass in the recurrent novae with a broad rectangular X-ray light curve shape.