X-ray observations of the neutron star in the Cas A supernova remnant over the past decade suggest the star is undergoing a rapid drop in surface temperature of \approx 2-5.5 % .
One explanation suggests the rapid cooling is triggered by the onset of neutron superfluidity in the core of the star , causing enhanced neutrino emission from neutron Cooper pair breaking and formation ( PBF ) .
Using consistent neutron star crust and core equations of state ( EOSs ) and compositions , we explore the sensitivity of this interpretation to the density dependence of the symmetry energy L of the EOS used , and to the presence of enhanced neutrino cooling in the bubble phases of crustal “ nuclear pasta ” .
Modeling cooling over a conservative range of neutron star masses and envelope compositions , we find L \lesssim 70 MeV , competitive with terrestrial experimental constraints and other astrophysical observations .
For masses near the most likely mass of M \gtrsim 1.65 M _ { \odot } , the constraint becomes more restrictive 35 \lesssim L \lesssim 55 MeV .
The inclusion of the bubble cooling processes decreases the cooling rate of the star during the PBF phase , matching the observed rate only when L \lesssim 45 MeV , taking all masses into consideration , corresponding to neutron star radii \lesssim 11 km .