INTEGRAL monitoring of unusually long X-ray bursts

TL;DR

This paper discusses INTEGRAL's monitoring of unusually long X-ray bursts from neutron stars, aiming to understand their nuclear ignition processes and relation to accretion states, especially at low accretion rates.

Contribution

It provides a comprehensive analysis of long X-ray bursts observed by INTEGRAL, highlighting their connection to accretion states and nuclear burning regimes.

Findings

Half of the observed intermediate long bursts were detected by INTEGRAL.

Long bursts are linked to specific accretion rates and compositions of accreted material.

The study sheds light on the transition from hydrogen-rich to helium-rich bursting regimes.

Abstract

X-ray bursts are thermonuclear explosions on the surface of accreting neutron stars in low mass X-ray binaries. As most of the known X-ray bursters are frequently observed by INTEGRAL, an international collaboration have been taking advantage of its instrumentation to specifically monitor the occurrence of exceptional burst events lasting more than ~10 minutes. Half of the so-called intermediate long bursts registered so far have been observed by INTEGRAL. The goal is to derive a comprehensive picture of the relationship between the nuclear ignition processes and the accretion states of the system leading up to such long bursts. Depending on the composition of the accreted material, these bursts may be explained by either the unstable burning of a large pile of mixed hydrogen and helium, or the ignition of a thick pure helium layer. Intermediate long bursts are particularly expected to occur at very low accretion rates and make possible to study the transition from a hydrogen-rich bursting regime to a pure helium regime.