Expanded atmospheres and winds in Type I X-ray bursts from accreting neutron stars

TL;DR

This paper models the transition from static envelopes to winds in neutron star atmospheres during type I X-ray bursts, revealing how photospheric radius and temperature evolve and impact observational interpretations.

Contribution

It introduces steady-state radiation-driven wind and envelope models incorporating general relativity, clarifying the conditions for observed photospheric radii during bursts.

Findings

Photospheric radius evolves from static envelopes to winds with increasing luminosity.

Most observed radius expansion bursts can be explained by static envelopes within a narrow luminosity range.

Spectral shifts are primarily due to gravitational redshift, with less than a few percent change.

Abstract

We calculate steady-state models of radiation-driven super-Eddington winds and static expanded envelopes of neutron stars caused by high luminosities in type I X-ray bursts. We use flux-limited diffusion to model the transition from optically thick to optically thin, and include effects of general relativity, allowing us to study the photospheric radius close to the star as the hydrostatic atmosphere evolves into a wind. We find that the photospheric radius evolves monotonically from static envelopes ($r_{\rm ph}\lesssim 50-70$ km) to winds ($r_{\rm ph}\approx 100-1000$ km). Photospheric radii of less than $100$ km, as observed in most photospheric radius expansion bursts, can be explained by static envelopes, but only in a narrow range of luminosity. In most bursts, we would expect the luminosity to increase further, leading to a wind with photospheric radius $\gtrsim 100$ km. In the contraction phase, the expanded envelope solutions show that the photosphere is still $\approx 1$ km above the surface when the effective temperature is only $3\%$ away from its maximum value. This is a possible systematic uncertainty when interpreting the measured Eddington fluxes from bursts at touchdown. We also discuss the applicability of steady-state models to describe the dynamics of bursts. In particular, we show that the sub to super-Eddington transition during the burst rise is rapid enough that static models are not appropriate. Finally, we analyze the strength of spectral shifts in our models. Expected shifts at the photosphere are dominated by gravitational redshift, and are therefore predicted to be less than a few percent.