Examining the Relationship Between the Persistent Emission and the Accretion Rate During a Type I X-ray Burst

TL;DR

This study uses simulations to investigate the relationship between the persistent emission normalization and accretion rate during Type I X-ray bursts, finding that the normalization mainly traces disk temperature, not accretion rate.

Contribution

It demonstrates through simulations that the persistent emission normalization $f_a$ is more closely related to disk temperature than to actual accretion rate changes during X-ray bursts.

Findings

$f_a$ correlates strongly with disk temperature.

$f_a$ weakly follows the mass accretion rate (correlation coefficient ≤ 0.63).

Heating increases disk emission at higher energies, reducing soft excess.

Abstract

The accretion flow onto a neutron star will be impacted due to irradiation by a Type I X-ray burst. The burst radiation exerts Poynting-Robertson (PR) drag on the accretion disk, leading to an enhanced mass accretion rate. Observations of X-ray bursts often find evidence that the normalization of the disk-generated persistent emission (commonly denoted by the factor $f_a$) increases during a burst, and changes in $f_a$ have been used to infer the evolution in the mass accretion rate due to PR drag. Here, we examine this proposed relationship between $f_a$ and mass accretion rate enhancement using time-resolved data from simulations of accretion disks impacted by Type I X-ray bursts. We consider bursts from both spinning and non-spinning neutron stars and track both the change in accretion rate due to PR grad and the disk emission spectra during the burst. Regardless of the neutron star spin, we find that $f_a$ strongly correlates with the disk temperature and only weakly follows the mass accretion rate (the Pearson correlation coefficients are $\leq 0.63$ in the latter case). Additionally, heating causes the disk to emit at higher energies, reducing its contribution to a soft excess. We conclude that $f_a$ cannot accurately capture the mass accretion rate enhancement and is rather a tracer of the disk temperature.