Urca cooling of the neutron star in the Cassiopeia A supernova remnant

Observed cooling rate of the young neutron star (NS) in the Cassiopeia A supernova remnant (Cas A NS) exceeds theoretical expectations based on conventional scenarios of NS cooling, controlled mainly by modified Urca (mUrca) neutrino emission. Several hypotheses have been suggested to explain these observations. The most popular one assumes the cooling enhancement by neutrino emission due to the Cooper pair breaking and formation (PBF) just after the onset of neutron superfluidity in the NS core. This explanation requires strict constraints on critical temperatures of proton and neutron superfluidities in the NS core and on the efficiency of the PBF cooling mechanism. These constraints are in tension with the modern theory. To relax them, Lev Leinson (2022) suggested a hybrid cooling scenario, where the direct Urca (dUrca) process of neutrino emission from a small NS central kernel contributes to the cooling enhancement in addition to the PBF process. We show that Cas A NS cooling needs not to be hybrid, as the joint effect of Urca (dUrca+mUrca) processes can explain the observations equally well with or without superfluidity and the PBF mechanism. We explore the Urca scenario with different assumptions about NS equation of state, baryon superfluidity, and composition of the outer heat-blanketing envelope. We show that the observed cooling rate can be reproduced with many combinations of these assumptions by tuning the NS mass, which should slightly exceed the threshold mass for opening the dUrca process in the kernel. Then the core stays non-isothermal for centuries, delaying the onset of enhanced dUrca cooling to satisfy the Cas A NS observations. In addition, we present an analytic toy model which elucidates many features of the Urca scenario.