Variability in the cycle length of the supersoft source RX J0513.9-6951

TL;DR

This paper analyzes 14 years of optical data from RX J0513.9-6951, revealing long-term cycle length variability that suggests significant changes in mass transfer rate, informing models of accretion wind evolution in supersoft X-ray binaries.

Contribution

It provides the first detailed analysis of long-term cycle variability in RX J0513.9-6951 and links these variations to changes in mass transfer rates within accretion wind models.

Findings

Cycle lengths and high state durations vary over years.

Variations imply mass transfer rate changes by a factor of 5.

Long-term modulation supports accretion wind evolution models.

Abstract

The supersoft X-ray binary RX J0513.9-6951 shows cyclic changes between optical-low / X-ray-on states and optical-high / X-ray-off states. It is supposed to be accreting close to the Eddington-critical limit and driven by "accretion wind evolution". We seek to derive the variations in the characteristic time scales of the long-term optical light curve and to determine the implications for the physical parameters of the system. We used existing and new optical monitoring observations covering a total time span of 14 years and compared the durations of the low and high states with the model calculations of Hachisu & Kato. The cycle lengths and especially the durations of the optical high states show a longterm modulation with variations that, according to the accretion wind evolution model, would imply variations in the mass transfer rate by a factor of 5 on timescales of years.