New models of photospheric radius expansion bursts

Simon Guichandut ( Université McGill )

Of the more than 7000 Type I X-ray bursts that have been observed, about a fifth of them exhibit photospheric radius expansion (PRE) as a result of near to super-Eddington luminosities being produced at the surface of the accreting neutron star (NS). PRE bursts are a key tool for high energy astrophysics as they contain a wealth of information about neutron star physics. We present recent and undergoing work on models of PRE bursts that go beyond previous simplifying approximations. We focus on a new study of the two regimes of PRE: sub-Eddington, where the atmosphere expands but remains in hydrostatic equilibrium, and super-Eddington, where a radiatively-driven wind expels accreted material and nuclear burning ashes from the star. We show that the existence of the former, apparent only once general relativistic effects are taken into account, may cause NS radius measurements from PRE to be over-estimated. For the latter, we discuss how our models can help infer the NS mass from gravitationally redshifted absorption lines of heavy elements ejected in the wind. These two aspects are relevant in the efforts to constrain the NS equation of state, upon which interpretations of other observations, such as multi-wavelength and gravitational wave observations of NS mergers, rely. Our models can also be coupled with time-dependent simulations of the NS burning layer to predict the light curve of these bursts in X-rays, especially relevant now with the advent of the NICER telescope, which is able to fully track the soft spectrum of PRE bursts. Finally, we demonstrate how the stellar evolution code MESA can be used to study PRE bursts that occur after the ignition of a stratified H/He envelope, a likely common scenario with a poorly understood observational signature, recently captured by NICER.