Type I X-ray Burst Emission Reflected into the Eclipses of EXO 0748-676

TL;DR

This study analyzes X-ray burst eclipses in the neutron star binary EXO 0748-676, revealing that observed in-eclipse bursts are not direct emissions but likely scattered or reflected, with implications for understanding accretion and outflows.

Contribution

It provides the first detailed analysis of Type I X-ray bursts occurring during eclipses, demonstrating that these are not direct emissions but involve scattering or reflection mechanisms.

Findings

In-eclipse bursts are obscured from direct view by the companion star.

Reflected flux from the accretion disc cannot fully explain the in-eclipse bursts.

Scattering by outflows or winds likely contributes to observed in-eclipse bursts.

Abstract

The neutron star X-ray binary, EXO 0748--676, was observed regularly by the Rossi X-ray Timing Explorer (RXTE) and XMM-Newton during its first detected outburst (1985 - 2008). These observations captured hundreds of asymmetric, energy-dependent X-ray eclipses, influenced by the ongoing ablation of the companion star and numerous Type I thermonuclear X-ray bursts. Here, we present the light curves of 22 Type I X-ray bursts observed by RXTE that coincide, fully or partially, with an X-ray eclipse. We identify nine instances where the burst occurs entirely within totality, seven bursts split across an egress, and six cases interrupted by an ingress. All in-eclipse bursts and split bursts occurred while the source was in the hard spectral state. We establish that we are not observing direct burst emission during eclipses since the companion star and the ablated outflow entirely obscure our view of the X-ray emitting region. We determine that the reflected flux from the outer accretion disc, even if maximally flared, is insufficient to explain all observations of in-eclipse X-ray bursts and instead explore scenarios whereby the emission arising from the X-ray bursts is scattered, either by a burst-induced rise in $N_{\rm{H}}$ that provides extra material, an accretion disc wind or the ablated outflow into our line of sight. However, the rarity of a burst and eclipse overlap makes it challenging to determine their origin.