Disc instability models for X-ray transients: evidence for evaporation and low -viscosity?

K Menou,J.-P Lasota,J.-M Hameury,R Narayan

doi:10.1046/j.1365-8711.2000.03357.x

K Menou, J.-P Lasota + Show 2 more

Open Access

PDF Available

https://doi.org/10.1046/j.1365-8711.2000.03357.x

Copy DOI

Export

Save

Cite

Abstract
Highlights/Summary
Full-Text PDF
Similar Papers

Abstract

Listen

We construct time-dependent models of accretion discs around black holes and neutron stars. We investigate the effect of evaporating the disc inner regions during quiescence on the predictions of the Disc Instability Model (DIM) for these systems. We do not include irradiation of the disc in the models. Removing the inner, most unstable parts of the accretion disc increases the predicted recurrence times. However, DIMs with values of the viscosity parameter alpha_hot ~ 0.1 and alpha_cold ~ 0.02 (typical of applications of the DIM to standard dwarf nova outbursts) fail to reproduce the long recurrence times of SXTs (unless we resort to fine-tuning of the parameters) independent of the evaporation strength. We show that models with evaporation and a smaller value of alpha_cold (~ 0.005) do reproduce the long recurrence times and the accretion rates at the level of the Eddington rate observed in outburst. The large difference between the values of alpha_hot and alpha_cold, if confirmed when disc irradiation is included, suggests that several viscosity mechanisms operate in these accretion discs. For some choices of parameters our models predict reflares during the decline from outburst. They are a physical property of the model and result from a heating front forming in the wake of an initial cooling front and subsequent, multiple front reflections. The reflares disappear in low-alpha models where front reflection can not occur.

Highlights

Low Mass Black Hole Binaries (LMBHBs) and Low Mass Neutron Stars Binaries (LMNSBs) are close binary systems in which a Roche-lobe filling, low mass secondary star transfers mass to a compact primary
We find that a TDIM with a small value of αcold (∼ 5 × 10−3) and strong evaporation does reproduce the long recurrence times and the high maximum values of the accretion rate in the discs of soft X-ray transients (SXTs)
In the Disc Instability Model (DIM), the inner regions of quiescent accretion discs are more subject to instabilities than the outer ones, so that the truncation of the inner disc has a stabilizing effect

Summary

Introduction

Low Mass Black Hole Binaries (LMBHBs) and Low Mass Neutron Stars Binaries (LMNSBs) are close binary systems in which a Roche-lobe filling, low mass (main sequence or sub-giant) secondary star transfers mass to a compact primary. Tanaka & Lewin 1995, van Paradijs & McClintock 1995, White, Nagase & Parmar 1995); these transient systems are known as soft X-ray transients (SXTs), or. Their outbursts last from several weeks to several months during which X–ray luminosities can approach the Eddington limit. SXTs are in some respects similar to dwarf novae (DN), which exhibit outbursts of 4 – 6 magnitudes in optical. These outbursts typically last days and are separated by periods of quiescence of a few weeks. DN are a subclass of Cataclysmic Variables (CVs; see Warner 1995 for a review), close binary systems similar to LMBHBs and LMNSBs, except for the compact primary which is a white dwarf

Results

Discussion

Conclusion