We perform the first multidimensional fluid simulations of thermonuclear helium ignition underneath
a hydrogen-rich shell. This situation is relevant to Type I X-ray bursts on neutron stars which accrete
from a hydrogen-rich companion. Using the low Mach number fluid code MAESTROeX, we investigate the
growth of the convection zone due to nuclear burning, and the evolution of the chemical abundances
in the atmosphere of the star. We also examine the convective boundary mixing processes which cause
the evolution to differ significantly from previous one-dimensional simulations that rely on mixinglength theory.
We find that the convection zone grows outwards as penetrating fluid elements cool
the overlying radiative layer, rather than directly from the increasing entropy of the convection zone
itself. Simultaneously, these flows efficiently mix composition, carrying carbon out of, and protons into
the convection zone even before contact with the hydrogen shell. We discuss the implications of these
effects for future modeling of these events and observations.
2023
ApJ
The Imprint of Convection on Type I X-Ray Bursts: Pauses in Photospheric Radius Expansion Lightcurves
Motivated by the recent observation by NICER of a type I X-ray burst from SAX J1808.4–3658 with a distinct
“pause” feature during its rise, we show that bursts which ignite in a helium layer underneath a hydrogen-rich shell
naturally give rise to such pauses, as long as enough energy is produced to eject the outer layers of the envelope by
super-Eddington winds. The length of the pause is determined by the extent of the convection generated after
ignition, while the rate of change of luminosity following the pause is set by the hydrogen gradient left behind by
convection. Using the MESA stellar evolution code, we simulate the accumulation, nuclear burning, and convective
mixing prior to and throughout the ignition of the burst, followed by the hydrodynamic wind. We show that the
results are sensitive to the treatment of convection adopted within the code. In particular, the efficiency of mixing
at the H/He interface plays a key role in determining the shape of the lightcurve. The data from
SAX J1808.4–3658 favor strong mixing scenarios. Multidimensional simulations will be needed to properly
model the interaction between convection and nuclear burning during these bursts, which will then enable a new
way to use X-ray burst lightcurves to study neutron star surfaces.
@article{guichandut2023,author={{Guichandut}, Simon and {Cumming}, Andrew},title={{The Imprint of Convection on Type I X-Ray Bursts: Pauses in Photospheric Radius Expansion Lightcurves}},journal={The Astrophysical Journal},keywords={Neutron stars},year={2023},month=aug,volume={954},number={1},pages={54},doi={10.3847/1538-4357/ace43c},adsurl={https://ui.adsabs.harvard.edu/abs/2023ApJ...954...54G},}
2021
ApJ
Expanded Atmospheres and Winds in Type I X-Ray Bursts from Accreting Neutron Stars
Simon Guichandut, Andrew Cumming, Maurizio Falanga, and
2 more authors
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 (rph ≲ 50-70 km) to winds (rph ≈ 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 ≳100 km. In the contraction phase, the expanded envelope solutions show that the photosphere is still ≈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.
@article{guichandut2021,author={{Guichandut}, Simon and {Cumming}, Andrew and {Falanga}, Maurizio and {Li}, Zhaosheng and {Zamfir}, Michael},title={Expanded Atmospheres and Winds in Type I X-Ray Bursts from Accreting Neutron Stars},journal={The Astrophysical Journal},publisher={apj},keywords={Neutron stars, Stellar winds, X-ray bursts, 1108, 1636, 1814, Astrophysics - High Energy Astrophysical Phenomena},year={2021},month=jun,volume={914},number={1},pages={49},doi={10.3847/1538-4357/abfa13},adsurl={https://ui.adsabs.harvard.edu/abs/2021ApJ...914...49G},}
2020
MSc Thesis
Regimes of photospheric radius expansion driven by high luminosities in Type I X-ray bursts