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

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 $β^{-}$ 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 $\sim4~{\rm GK}$, 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.

Figures (4)

• Figure 1: Neutrino luminosity of each process as a function of temperature. See texts for details.
• Figure 2: Cooling curves without and with Urca cycles in the ocean, denoting black and red solid lines, for $M_{\rm core}=1.18 M_\odot$ and $R_{\rm core}=7.96~{\rm km}$ stars. The shadow region corresponds to the Eddington flux, including uncertainties of the hydrogen mass fraction.
• Figure 3: Evolution of temperature structure without and with Urca cycles in the ocean, denoting dashed and solid lines, respectively, for $M_{\rm core}=1.18 M_\odot$ and $R_{\rm core}=7.96~{\rm km}$ stars.
• Figure 4: The log-likelihood distribution in the mass–radius plane obtained from the MCMC analysis. Blue contours corresponding to $\ln\mathcal{L}=-4$, -6, and -8 are shown, although no contours with $\ln\mathcal{L}\ge -6$ appear in panel (b).