Abstract
The disk instability picture gives a plausible explanation for the behavior of soft X-ray transient systems if self-irradiation of the disk is included. We show that there is a simple relation between the peak luminosity (at the start of an outburst) and the decay timescale. We use this relation to place constraints on systems assumed to undergo disk instabilities. The observable X-ray populations of elliptical galaxies must largely consist of long-lived transients, as deduced on different grounds by Piro and Bildsten (2002). The strongly-varying X-ray source HLX-1 in the galaxy ESO 243-49 can be modeled as a disk instability of a highly super-Eddington stellar-mass binary similar to SS433. A fit to the disk instability picture is not possible for an intermediate-mass black hole model for HLX-1. Other, recently identified, super-Eddington ULXs might be subject to disk instability.
Highlights
Many bright X-ray sources are strongly variable
We show here that there is a simple connection between the accretor mass, the peak luminosity at outburst, and the decay time of the outburst, if this is well-defined by observations
From Equation (11) this requires a decay time of the order of decades or centuries, in complete contrast to the observed 180 days. This supports the conclusion of Lasota et al (2011), who found that a disk instability model could not explain the light curve of HLX-1 if the system was assumed to contain an intermediate-mass black hole (IMBH) and be at 95 Mpc
Summary
Many bright X-ray sources are strongly variable. It is largely accepted that much of this variability results from the thermal–viscous disk instability (see Lasota 2001 for a review). Irradiated disk accreting at constant rate of ~3Mc+rit (Rd ), where Mcrit (Rd ) is the value of the minimum critical accretion for a hot irradiated disk at its outer radius Rd (see, e.g., Figure 31 in Lasota 2001). This fixes the critical accretion rate Mmax through the relation (Lasota et al 2008). The second observable property is the decay time for the X-rays: in soft X-ray transients (unlike dwarf novae), disk irradiation by the central X-rays traps the disk in the hot high state and only allows a decay of Mon the hot-state viscous timescale (King & Ritter 1998; Dubus et al 2001)
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