  Transiently accreting neutron stars in quiescence (Lx<10^34 erg/s) have been
observed to vary in intensity by factors of few, over timescales of days to
years. If the quiescent luminosity is powered by a hot NS core, the core
cooling timescale is much longer than the recurrence time, and cannot explain
the observed, more rapid variability. However, the non-equilibrium reactions
which occur in the crust during outbursts deposit energy in iso-density shells,
from which the thermal diffusion timescale to the photosphere is days to years.
The predicted magnitude of variability is too low to explain the observed
variability unless - as is widely believed - the neutrons beyond the
neutron-drip density are superfluid. Even then, variability due to this
mechanism in models with standard core neutrino cooling processes is less than
50 per cent - still too low to explain the reported variability. However,
models with rapid core neutrino cooling can produce variability by a factor as
great as 20, on timescales of days to years following an outburst. Thus, the
factors of few intensity variability observed from transiently accreting
neutron stars can be accounted for by this mechanism only if rapid core cooling
processes are active.
