Crust cooling curves of accretion-heated neutron stars

We discuss the recent efforts to use a sub-class of neutron-star X-ray transients (the quasi-persistent transients) to probe the properties of neutron-star crusts and cores. Quasi-persistent X-ray transients experience accretion episodes lasting years to decades, instead of the usual weeks to months of ordinary, shortduration transients. These prolonged accretion episodes should significantly heat the crusts of the neutron stars in these systems, bringing the crusts out of thermal equilibrium with their neutron-star cores. When these systems are back in quiescence, i.e. when no more accretion onto the neutron-star surfaces occurs, then the crusts should thermally radiate in X-rays, cooling them down until they are again in thermal equilibrium with the cores. In this chapter we discuss the recent X-ray monitoring campaigns we performed (using the X-ray satellites Chandra and XMM-Newton) to study several quasi-persistent neutron-star X-ray transients in their quiescent states. These observations gave us, for the first time, a detailed look into the cooling curves of accretion heated neutron-star crusts. In this chapter, we discuss how these crust cooling curves can provide insight into the structure of neutron stars.