Evolution of Urca Pairs in the Crusts of Highly Magnetized Neutron Stars

The effects of strong magnetic fields on neutrino emission in the modified Urca process are studied. Landau levels on the various Urca pairs affects the neutrino emission spectrum and leads to an angular asymmetry in the neutrino emission on neutron star surfaces. For low magnetic fields the Landau levels have almost no effect on the cooling. However, as the field strength increases, the electron chemical potential increases resulting in a lower density at which Urca pairs can exist. For intermediate field strength there is an interesting interference between the Landau level distribution and the Fermi distribution. For high enough field strength, the entire electron energy spectrum is eventually confined to single Landau level producing dramatic spikes in the emission spectrum. As a result, neutrino emissivity depends on polar angle on the stellar surface with respect to the magnetic poles. The overall luminosity is affected. In addition, because the electron phase space is altered in the presence of a magnetic field Urca pairs may be prohibited in a magnetic field, new Urca pairs may emerge, and Urca pairs involving highly-excited states may evolve.