  Modelling of crust heating and cooling across multiple accretion outbursts of
the low mass X-ray binary MXB 1659-29 indicates that the neutrino luminosity of
the neutron star core is consistent with direct Urca reactions occurring in
$\sim 1\%$ of the core volume. We investigate this scenario with neutron star
models that include a detailed equation of state parametrized by the slope of
the nuclear symmetry energy $L$, and a range of neutron and proton superfluid
gaps. We find that the predicted neutron star mass depends sensitively on $L$
and the assumed gaps. We discuss which combinations of superfluid gaps
reproduce the inferred neutrino luminosity. Larger values of $L\gtrsim 80\ {\rm
MeV}$ require superfluidity to suppress dUrca reactions in low mass neutron
stars, i.e. that the proton or neutron gap is sufficiently strong and extends
to high enough density. However, the largest gaps give masses near the maximum
mass, making it difficult to accommodate colder neutron stars. We consider
models with reduced dUrca normalization as an approximation of alternative,
less efficient, fast cooling processes in exotic cores. We find solutions with
a larger emitting volume, providing a more natural explanation for the observed
neutrino luminosity, provided the fast cooling process is within a factor of
$\sim 1000$ of dUrca. The heat capacities of our models span the range from
fully-paired to fully-unpaired nucleons meaning that long term observations of
core cooling could distinguish between models. We discuss the impact of future
constraints on neutron star mass, radius and the density dependence of the
symmetry energy.
