Rapid neutrino cooling in the neutron star MXB 1659-29

TL;DR

This paper demonstrates that the neutron star in MXB 1659-29 cools rapidly via direct Urca neutrino processes, indicating a high core neutrino luminosity and providing insights into the star's internal composition and cooling mechanisms.

Contribution

It provides the first firm observational evidence of fast neutrino cooling in a neutron star with a detectable thermal component in its X-ray spectrum.

Findings

Core neutrino luminosity exceeds modified Urca predictions.

Direct Urca reactions occur in a small core fraction.

Neutron star core likely has unpaired nucleons and high proton fraction.

Abstract

We show that the neutron star in the transient system MXB~1659-29 has a core neutrino luminosity that substantially exceeds that of the modified Urca reactions (i.e., $n+n\to n+p+e^{-}+\bar{\nu}_{e}$ and inverse) and is consistent with the direct Urca reactions ($n\to p+e^{-}+\bar{\nu}_{e}$ and inverse) occurring in a small fraction of the core. Observations of the thermal relaxation of the neutron star crust following 2.5 years of accretion allow us to measure the energy deposited into the core during accretion, which is then reradiated as neutrinos, and infer the core temperature. For a nucleonic core, this requires that the nucleons are unpaired and that the proton fraction exceed a critical value to allow the direct Urca reaction to proceed. The neutron star in MXB~1659-29 is the first with a firmly detected thermal component in its X-ray spectrum that needs a fast neutrino cooling process. Measurements of the temperature variation of the neutron star core during quiescence would place an upper limit on the core specific heat and serve as a check on the fraction of the neutron star core in which nucleons are unpaired.