Outbursts in ultracompact X-ray binaries

TL;DR

This paper explores the thermal-viscous instability in hydrogen-poor accretion discs of ultracompact X-ray binaries to explain their faint transient outbursts, matching observed properties by adjusting disc viscosity parameters.

Contribution

It models the instability in hydrogen-poor discs, incorporating X-ray irradiation and mass-transfer effects, to explain faint and bright transient behaviors in ultracompact binaries.

Findings

Reproduces observed outburst luminosities, durations, and recurrence times.

Suggests lower viscosity in quiescence due to chemical composition effects.

Highlights the role of non-ideal MHD effects in poorly ionized discs.

Abstract

Very faint X-ray binaries appear to be transient in many cases with peak luminosities much fainter than that of usual soft X-ray transients, but their nature still remains elusive. We investigate the possibility that this transient behaviour is due to the same thermal/viscous instability which is responsible for outbursts of bright soft X-ray transients, occurring in ultracompact binaries for adequately low mass-transfer rates. More generally, we investigate the observational consequences of this instability when it occurs in ultracompact binaries. We use our code for modelling the thermal-viscous instability of the accretion disc, assumed here to be hydrogen poor. We also take into account the effects of disc X-ray irradiation, and consider the impact of the mass-transfer rate on the outburst brightness. We find that one can reproduce the observed properties of both the very faint and the brighter short transients (peak luminosity, duration, recurrence times), provided that the viscosity parameter in quiescence is slightly smaller (typically a factor of between two and four) than in bright soft X-ray transients and normal dwarf nova outbursts, the viscosity in outburst being unchanged. This possibly reflects the impact of chemical composition on non-ideal MHD effects affecting magnetically driven turbulence in poorly ionized discs.