Impact of neutron star spin on Poynting-Robertson drag during a Type I X-ray burst

TL;DR

This study investigates how neutron star spin influences Poynting-Robertson drag during Type I X-ray bursts, showing that spin can enhance the effect and support its role in observed spectral features.

Contribution

It demonstrates that neutron star spin can strengthen Poynting-Robertson drag effects during X-ray bursts, challenging previous assumptions about spin weakening the phenomenon.

Findings

PR drag moves inner disc outward by ~1.6-2.2 km during bursts.

Mass accretion rate increases by ~7.9-11.2 times during bursts.

Spin can enhance PR drag effects despite expectations to the contrary.

Abstract

External irradiation of a neutron star (NS) accretion disc induces Poynting-Robertson (PR) drag, removing angular momentum and increasing the mass accretion rate. Recent simulations show PR drag significantly enhancing the mass accretion rate during Type I X-ray bursts, which could explain X-ray spectral features such as an increase in the persistent emission and a soft excess. However, prograde spin of the NS is expected to weaken PR drag, challenging its importance during bursts. Here, we study the effect of spin on PR drag during X-ray bursts. We run four simulations, with two assuming a non-spinning NS and two using a spin parameter of $a_*=0.2$, corresponding to a rotation frequency of 500 Hz. For each scenario, we simulate the disc evolution subject to an X-ray burst and compare it to the evolution found with no burst. PR drag drains the inner disc region during a burst, moving the inner disc radius outward by $\approx1.6$ km in the $a_*=0$ and by $\approx2.2$ km in the $a_*=0.2$ simulation. The burst enhances the mass accretion rate across the innermost stable circular orbit $\approx7.9$ times when the NS is not spinning and $\approx11.2$ times when it is spinning. The explanation for this seemingly contradictory result is that the disc is closer to the NS when $a_*=0.2$, and the resulting stronger irradiating flux offsets the weakening effect of spin on the PR drag. Hence, PR drag remains a viable explanation for the increased persistent emission and soft excess observed during X-ray bursts in spinning NS systems.