Puzzling thermonuclear burst behaviour from the transient low-mass X-ray binary IGR J17473-2721

TL;DR

This study analyzes 57 thermonuclear bursts from the transient X-ray binary IGR J17473-2721 during its 2008 outburst, revealing complex burst behavior and potential thermal effects influencing nuclear burning processes.

Contribution

It provides the first detailed analysis of burst behavior across a wide range of accretion rates in this source, highlighting phases and hysteresis in burst activity.

Findings

Burst rate increased with accretion rate then dropped before outburst peak.

Burst activity resumed at a lower rate after the peak.

Hysteresis in burst behavior suggests thermal effects on nuclear burning.

Abstract

We investigate the thermonuclear bursting behaviour of IGR J17473-2721, an X-ray transient that in 2008 underwent a six month long outburst, starting (unusually) with an X-ray burst. We detected a total of 57 thermonuclear bursts throughout the outburst with AGILE, Swift, RXTE, and INTEGRAL. The wide range of inferred accretion rates (between <1% and about 20% of the Eddington accretion rate m-dot_Edd) spanned during the outburst allows us to study changes in the nuclear burning processes and to identify up to seven different phases. The burst rate increased gradually with the accretion rate until it dropped (at a persistent flux corresponding to about 15% of m-dot_Edd) a few days before the outburst peak, after which bursts were not detected for a month. As the persistent emission subsequently decreased, the bursting activity resumed with a much lower rate than during the outburst rise. This hysteresis may arise from the thermal effect of the accretion on the surface nuclear burning processes, and the timescale is roughly consistent with that expected for the neutron star crust thermal response. On the other hand, an undetected superburst, occurring within a data gap near the outburst peak, could have produced a similar quenching of burst activity.