Multi-Instrument X-ray Observations of Thermonuclear Bursts with Short Recurrence Times

TL;DR

This study analyzes a large catalog of X-ray bursts from low-mass X-ray binaries, revealing that short recurrence times are linked to hydrogen burning and rapid neutron star spins, with some bursts occurring as soon as 3.8 minutes apart.

Contribution

It provides the first comprehensive assessment of short recurrence time X-ray bursts using an extensive observational catalog and identifies key factors influencing their occurrence.

Findings

Short recurrence bursts are not observed in ultra-compact binaries.

All sources with short recurrence bursts spin faster than 500 Hz.

The shortest observed recurrence time is 3.8 minutes.

Abstract

Type I X-ray bursts from low-mass X-ray binaries result from a thermonuclear runaway in the material accreted onto the neutron star. Although typical recurrence times are a few hours, consistent with theoretical ignition model predictions, there are also observations of bursts occurring as promptly as ten minutes or less after the previous event. We present a comprehensive assessment of this phenomenon using a catalog of 3387 bursts observed with the BeppoSAX/WFCs and RXTE/PCA X-ray instruments. This catalog contains 136 bursts with recurrence times of less than one hour, that come in multiples of up to four events, from 15 sources. Short recurrence times are not observed from so-called ultra-compact binaries, indicating that hydrogen burning processes play a crucial role. As far as the neutron star spin frequency is known, these sources all spin fast at over 500 Hz; the rotationally induced mixing may explain burst recurrence times of the order of 10 min. Short recurrence time bursts generally occur at all mass accretion rates where normal bursts are observed, but for individual sources the short recurrence times may be restricted to a smaller interval of accretion rate. The fraction of such bursts is roughly 30%. We also report the shortest known recurrence time of 3.8 minutes.