Time-dependent, compositionally driven convection in the oceans of accreting neutron stars

TL;DR

This paper models how chemical separation and convective mixing in accreting neutron star oceans affect temperature and composition during outbursts and quiescence, predicting observable light curve features.

Contribution

It extends steady-state models to time-dependent scenarios, showing rapid convective mixing and its impact on thermal and compositional evolution in neutron star oceans.

Findings

Convective mixing flattens composition profiles within weeks to months.

Inward heat transport during quiescence causes rapid cooling of outer layers.

Predicted surface emission dips are observable within a week after accretion ends.

Abstract

We discuss the effect of chemical separation as matter freezes at the base of the ocean of an accreting neutron star, and the subsequent enrichment of the ocean in light elements and inward transport of heat through convective mixing. We extend the steady-state results of Medin & Cumming 2011 to transiently accreting neutron stars, by considering the time-dependent cases of heating during accretion outbursts and cooling during quiescence. Convective mixing is extremely efficient, flattening the composition profile in about one convective turnover time (weeks to months at the base of the ocean). During accretion outbursts, inward heat transport has only a small effect on the temperature profile in the outer layers until the ocean is strongly enriched in light elements, a process that takes hundreds of years to complete. During quiescence, however, inward heat transport rapidly cools the outer layers of the ocean while keeping the inner layers hot. We find that this leads to a sharp drop in surface emission at around a week followed by a gradual recovery as cooling becomes dominated by the crust. Such a dip should be observable in the light curves of these neutron star transients, if enough data is taken at a few days to a month after the end of accretion. If such a dip is definitively observed, it will provide strong constraints on the chemical composition of the ocean and outer crust.