Time-variable emission from transiently accreting neutron stars in quiescence due to deep crustal heating

Transiently accreting neutron stars in quiescence (L-X less than or similar to 10(34) erg s(-1)) have been observed to vary in intensity by factors of few, over time-scales of days to years. If the quiescent luminosity is pow ered by a hot neutron star core, the core cooling time-scale is much longer than the recurrence time, and cannot explain the observed, more rapid vari ability. However, the non-equilibrium reactions which occur in the crust du ring outbursts deposit energy in isodensity shells, from which the thermal diffusion time-scale to the photosphere is days to years. The predicted mag nitude of variability is too low to explain the observed variability unless -as is widely believed-the neutrons beyond the neutron-drip density are sup erfluid. Even then, the variability due to this mechanism in models with st andard core neutrino cooling processes is less than 50 per cent-still too l ow to explain the reported variability. However, models with rapid core neu trino cooling can produce a variability by a factor as great as 20, on time -scales of days to years following an outburst. Thus, the factors of simila r to few intensity variability observed from transiently accreting neutron stars can be accounted for by this mechanism only if rapid core cooling pro cesses are active.