Evidence of Nuclear Urca Process in the Ocean of Neutron-Star Superburst MAXI J1752$-$457

TL;DR

This paper presents evidence that the rapid cooling observed in a neutron star superburst is caused by a nuclear Urca process involving electron capture and beta decay in the star's ocean, marking the first such indication.

Contribution

It provides the first observational evidence of the nuclear Urca process occurring in the neutron star ocean during a superburst.

Findings

Rapid cooling over 4 days attributed to neutrino emission from Urca pairs.

Identification of a hot ignition layer near the Urca shell with ~4 GK temperature.

Distinct behavior from normal Type-I X-ray bursts, indicating different burning layers.

Abstract

We propose that the rapid cooling of the neutron star following its X-ray superburst in MAXI J1752$-$457 over a period of 4 days, observed by two Japanese satellites, MAXI and NinjaSat, is due to enhanced neutrino emission from cycles of electron capture and $\beta^{-}$ decay involving odd-$A$ nuclei (or Urca pairs) in the ocean. Hence, this work provides the first indication of the possible existence of such a ``nuclear Urca process". The observation of MAXI J1752$-$457 implies a hot ignition layer with a maximum temperature of $\sim4~{\rm GK}$, located near the Urca shell in the ocean, such that the nuclear Urca process becomes dominant for up to $\sim2$ days after the superburst. This behavior is distinct from that of normal Type-I X-ray bursts, which are triggered by hydrogen or helium burning in much shallower layers than those of superbursts. Our findings enable probing of superburst ashes through Urca pairs via long-term monitoring of crust cooling on day-long timescales.