Multiple X-ray bursts and the model of a spreading layer of accreting matter over the neutron star surface

TL;DR

This paper explains the occurrence of rapid series of X-ray bursts on neutron stars using a spreading layer model of accreting matter, highlighting the need to refine the model to account for triple bursts.

Contribution

It introduces a refined spreading layer model that accounts for multiple X-ray bursts, including triple bursts, at high accretion rates on neutron stars.

Findings

Series of close X-ray bursts are explained by the spreading layer model.

Triple bursts suggest the existence of a central ring zone in the model.

High accretion rates are necessary for the observed burst series.

Abstract

We report the detection during the JEM-X/INTEGRAL observations of several X-ray bursters of series of close type I X-ray bursts consisting of two or three events with a recurrence time much shorter than the characteristic (at the observed mean accretion rate) time of matter accumulation needed for a thermonuclear explosion to be initiated on the neutron star surface. We show that such series of bursts are naturally explained in the model of a spreading layer of accreting matter over the neutron star surface in the case of a sufficiently high ($\dot{M}\geq 1\times 10^{-9}\ M_{\odot}\ \mbox{yr}^{-1}$) accretion rate (corresponding to a mean luminosity $L_{\rm tot}\geq 1\times 10^{37} \mbox{erg s}^{-1}$). The existence of triple bursts requires some refinement of the model - the importance of a central ring zone is shown. In the standard model of a spreading layer no infall of matter in this zone is believed to occur.