Simulating the Collapse of a Thick Accretion Disk due to a Type I X-ray Burst from a Neutron Star

TL;DR

This study uses advanced simulations to explore how intense Type I X-ray bursts cause significant cooling and structural changes in accretion disks around neutron stars, impacting our understanding of neutron star properties.

Contribution

It presents the first detailed 2D general relativistic radiation hydrodynamic simulations of disk response to X-ray bursts, highlighting cooling effects and Poynting-Robertson drag.

Findings

Disk temperature drops by over three orders of magnitude during bursts.

Scale height of the disk decreases by more than an order of magnitude.

Poynting-Robertson drag increases accretion rate by a factor of 3-4.

Abstract

We use two-dimensional, general relativistic, viscous, radiation hydrodynamic simulations to study the impact of a Type I X-ray burst on a hot and geometrically thick accretion disk surrounding an unmagnetized, non-rotating neutron star. The disk is initially consistent with a system in its low/hard spectral state, and is subject to a burst which rises to a peak luminosity of $10^{38}$ erg s$^{-1}$ in $2.05$ s. At the peak of the burst, the temperature of the disk has dropped by more than three orders of magnitude and its scale height has gone down by more than one order of magnitude. The simulations show that these effects predominantly happen due to Compton cooling of the hot plasma, and clearly illustrate the potential cooling effects of bursts on accretion disk coronae. In addition, we demonstrate the presence of Poynting-Robertson drag, though it only enhances the mass accretion rate onto the neutron star by a factor of $\sim 3$-$4$ compared to a simulation with no burst. Simulations such as these are important for building a general understanding of the response of an accretion disk to an intense X-ray impulse, which, in turn, will be crucial for deciphering burst spectra. Detailed analysis of such spectra offers the potential to measure neutron star radii, and hence constrain the neutron star equation of state, but only if the contributions coming from the impacted disk and its associated corona can be understood.