Probing the effects of a thermonuclear X-ray burst on the neutron star accretion flow with NuSTAR

TL;DR

This study uses NuSTAR observations to analyze how a thermonuclear X-ray burst affects the accretion flow around a neutron star, revealing significant emission enhancement and spectral changes during the burst.

Contribution

It demonstrates the effectiveness of non-negative matrix factorization in characterizing accretion emission evolution during X-ray bursts, providing new insights into accretion dynamics.

Findings

Accretion emission increased up to five times during the burst.

NMF effectively characterizes burst evolution and spectral changes.

A small pre-burst increase in accretion emission was observed.

Abstract

Observational evidence has been accumulating that thermonuclear X-ray bursts ignited on the surface of neutron stars influence the surrounding accretion flow. Here, we exploit the excellent sensitivity of NuSTAR up to 79 keV to analyze the impact of an X-ray burst on the accretion emission of the neutron star LMXB 4U 1608-52. The ~200 s long X-ray burst occurred during a hard X-ray spectral state, and had a peak intensity of ~30-50 per cent of the Eddington limit with no signs of photospheric radius expansion. Spectral analysis suggests that the accretion emission was enhanced up to a factor of ~5 during the X-ray burst. We also applied a linear unsupervised decomposition method, namely non-negative matrix factorization (NMF), to study this X-ray burst. We find that the NMF performs well in characterizing the evolution of the burst emission and is a promising technique to study changes in the underlying accretion emission in more detail than is possible through conventional spectral fitting. For the burst of 4U 1608-52, the NMF suggests a possible softening of the accretion spectrum during the X-ray burst, which could potentially be ascribed to cooling of a corona. Finally, we report a small (~3 per cent) but significant rise in the accretion emission ~0.5 h before the X-ray burst, although it is unclear whether this was related to the X-ray burst ignition.