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

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 β- 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 about 4 GK, located near the Urca shell in the ocean, such that the nuclear Urca process becomes dominant for up to about 2 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.