Evidence for enhanced persistent emission during sub-Eddington thermonuclear bursts

TL;DR

This study shows that persistent X-ray emission often increases during thermonuclear bursts, challenging previous assumptions, and introduces a modified spectral analysis method that accounts for this variability.

Contribution

We developed a spectral analysis approach incorporating a variable persistent emission factor, revealing increased accretion during bursts and improving fit quality.

Findings

Persistent emission typically increases during bursts.

Elevated $f_a$ correlates inversely with persistent flux.

Modified model yields better spectral fits.

Abstract

The standard approach for time-resolved X-ray spectral analysis of thermonuclear bursts involves subtraction of the pre-burst emission as background. This approach implicitly assumes that the persistent flux remains constant throughout the burst. We reanalyzed 332 photospheric radius expansion bursts observed from 40 sources by the Rossi X-ray Timing Explorer, introducing a multiplicative factor $f_a$ to the persistent emission contribution in our spectral fits. We found that for the majority of spectra the best-fit value of $f_a$ is significantly greater than 1, suggesting that the persistent emission typically increases during a burst. Elevated $f_a$ values were not found solely during the radius expansion interval of the burst, but were also measured in the cooling tail. The modified model results in a lower average value of the $\chi^2$ fit statistic, indicating superior spectral fits, but not yet to the level of formal statistical consistency for all the spectra. We interpret the elevated $f_a$ values as an increase of the mass accretion rate onto the neutron star during the burst, likely arising from the effects of Poynting-Robertson drag on the disk material. We measured an inverse correlation of $f_a$ with the persistent flux, consistent with theoretical models of the disc response. We suggest that this modified approach may provide more accurate burst spectral parameters, as well as offering a probe of the accretion disk structure.