Outbursts in ultracompact X-ray binaries

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.

9 pages, 7 figures, Astronomy and Astrophysics, in press

References in corpus (13)

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• Long-term luminosity behavior of 14 ultracompact X-ray binaries
• Anomalous Z Cam stars: a response to mass-transfer outbursts
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• Mass transfer variation in the outburst model of dwarf novae and soft X-ray transients
• Hydrodynamic simulations of irradiated secondaries in dwarf novae

Cited by in corpus (5)

• Accretion Flows in Nonmagnetic White Dwarf Binaries as Observed in X-rays
• New insights on the puzzling LMXB 1RXS J180408.9-342058: the intermediate state, the clocked type-I X-ray bursts and much more
• Near-infrared counterparts of three transient very faint neutron star X-ray binaries
• MAXI J1957+032: an accreting neutron star possibly in a triple system
• Constraining the duty cycle of transient low-mass X-ray binaries through simulations