X-ray Transients: Hyper- or Hypo-Luminous?

TL;DR

This paper explores the behavior of X-ray transients through disk instability models, linking peak luminosity to decay timescale, and discusses implications for different types of X-ray sources including ULXs and HLX-1.

Contribution

It demonstrates a simple relation between peak luminosity and decay timescale in X-ray transients and applies this to constrain the nature of various X-ray sources.

Findings

Observable X-ray populations in elliptical galaxies are mainly long-lived transients.

HLX-1 can be modeled as a super-Eddington stellar-mass binary disk instability.

Disk instability models do not fit an intermediate-mass black hole scenario for HLX-1.

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.