Revisiting a fundamental test of the disc instability model for X-ray binaries

TL;DR

This study tests the disc instability model (DIM) for X-ray binaries by comparing observed system stability with theoretical predictions, finding strong overall agreement despite some individual discrepancies.

Contribution

It provides empirical validation of the DIM's critical mass transfer rate prediction using a large sample of X-ray binaries and introduces the transientness parameter as a new activity measure.

Findings

Observed persistent and transient systems align with the model's stability criteria.

The transientness parameter correlates with outburst recurrence time as predicted.

Overall, the DIM successfully explains long-term average behavior of X-ray binaries.

Abstract

We revisit a core prediction of the disc instability model (DIM) applied to X-ray binaries. The model predicts the existence of a critical mass transfer rate, which depends on disc size, separating transient and persistent systems. We therefore selected a sample of 52 persistent and transient neutron star and black hole X-ray binaries and verified if observed persistent (transient) systems do lie in the appropriate stable (unstable) region of parameter space predicted by the model. We find that, despite the significant uncertainties inherent to these kinds of studies, the data are in very good agreement with the theoretical expectations. We then discuss some individual cases that do not clearly fit into this main conclusion. Finally, we introduce the transientness parameter as a measure of the activity of a source and show a clear trend of the average outburst recurrence time to decrease with transientness in agreement with the DIM predictions. We therefore conclude that, despite difficulties in reproducing the complex details of the lightcurves, the DIM succeeds to explain the global behaviour of X-ray binaries averaged over a long enough period of time.