Variability in the thermal emission from accreting neutron star transients

The composition of the outer 100 m of a neutron star sets the heat flux that flows outward from the core. For an accreting neutron star in an X-ray transient, the thermal quiescent flux depends sensitively on the amount of hydrogen and helium remaining on the surface after an accretion outburst and on the composition of the underlying ashes of previous H/He-4 burning. Because H/He-4 has a higher thermal conductivity, a larger mass of H/He-4 implies a shallower thermal gradient through the low-density envelope and hence a higher effective temperature for a given core temperature. The mass of residual H and He-4 varies from outburst to outburst, so the thermal quiescent flux is variable even though the core temperature is constant for timescales less than or similar to10(4) yr. Heavy elements settle from an H/He-4 envelope in a few hours; we therefore model the quiescent envelope as two distinct layers, H/He-4 over heavier elements, and treat the mass of H/He-4 as a free parameter. We find that the emergent thermal quiescent flux can vary by a factor of 2-3 between different quiescent epochs. The variation is more pronounced at lower interior temperatures, making systems with low quiescent luminosities and frequent outbursts, such as SAX J1808.4-3658, ideal candidates from which to observe this effect. Because the ashes of H/He-4 burning are heavier than Fe-56, their thermal conductivity is greatly reduced. This increases the inferred crust temperature beyond previous estimates for a given effective temperature. We survey this effect for different ash compositions and apply our calculations to Cen X-4, Aql X-1, and SAX J1808.4-3658. In the case of Aql X-1, the inferred high interior temperature suggests that neutrino cooling contributes to the neutron star's thermal balance.