Rotochemical heating in millisecond pulsars: modified Urca reactions with uniform Cooper pairing gaps

TL;DR

This paper investigates how uniform superfluid energy gaps in neutron and proton components affect rotochemical heating in millisecond pulsars, revealing increased thresholds for chemical imbalance and longer times to reach thermal equilibrium.

Contribution

It introduces a model incorporating uniform Cooper pairing gaps into rotochemical heating, showing their impact on thermal evolution and luminosity in millisecond pulsars.

Findings

Superfluid gaps raise the chemical imbalance threshold beyond the non-superfluid case.

Superfluid MSPs take longer to reach quasi-steady state and have higher luminosity.

The model explains UV emission of PSR J0437-4715 with specific gap values.

Abstract

Context: When a rotating neutron star loses angular momentum, the reduction in the centrifugal force makes it contract. This perturbs each fluid element, raising the local pressure and originating deviations from beta equilibrium that enhance the neutrino emissivity and produce thermal energy. This mechanism is named rotochemical heating and has previously been studied for neutron stars of nonsuperfluid matter, finding that they reach a quasi-steady configuration in which the rate at which the spin-down modifies the equilibrium concentrations is the same at which neutrino reactions restore the equilibrium. Aims: We describe the thermal effects of Cooper pairing with spatially uniform energy gaps of neutrons \Delta_n and protons \Delta_p on the rotochemical heating in millisecond pulsars (MSPs) when only modified Urca reactions are allowed. By this, we may determine the amplitude of the superfluid energy gaps for the neutron and protons needed to produce different thermal evolution of MSPs. Results: We find that the chemical imbalances in the star grow up to the threshold value \Delta_{thr}= min(\Delta_n+ 3\Delta_p, 3\Delta_n+\Delta_p), which is higher than the quasi-steady state achieved in absence of superfluidity. Therefore, the superfluid MSPs will take longer to reach the quasi-steady state than their nonsuperfluid counterparts, and they will have a higher a luminosity in this state, given by L_\gamma ~ (1-4) 10^{32}\Delta_{thr}/MeV \dot{P}_{-20}/P_{ms}^3 erg s^-1. We can explain the UV emission of the PSR J0437-4715 for 0.05 MeV<\Delta_{thr}<0.45 MeV.