A "Hyperburst" in the MAXI J0556-332 Neutron Star: Evidence for a New Type of Thermonuclear Explosion

TL;DR

This paper presents evidence for a rare, massive thermonuclear hyperburst in a neutron star, inferred from cooling data and modeling, marking the first observational indication of such an event.

Contribution

It introduces the concept of a hyperburst in a neutron star, supported by numerical simulations and observational data, revealing a new type of thermonuclear explosion in neutron star crusts.

Findings

Evidence for a hyperburst event in MAXI J0556-332

Hyperburst released 10^44 ergs at high density

Hyperburst's imprint observed in neutron star cooling

Abstract

The study of transiently accreting neutron stars provides a powerful means to elucidate the properties of neutron star crusts. We present extensive numerical simulations of the evolution of the neutron star in the transient low-mass X-ray binary MAXI J0556--332. We model nearly twenty observations obtained during the quiescence phases after four different outbursts of the source in the past decade, considering the heating of the star during accretion by the deep crustal heating mechanism complemented by some shallow heating source. We show that cooling data are consistent with a single source of shallow heating acting during the last three outbursts, while a very different and powerful energy source is required to explain the extremely high effective temperature of the neutron star, ~350 eV, when it exited the first observed outburst. We propose that a gigantic thermonuclear explosion, a "hyperburst" from unstable burning of neutron rich isotopes of oxygen or neon, occurred a few weeks before the end of the first outburst, releasing 10^44 ergs at densities of the order of 10^11 g/cm^3. This would be the first observation of a hyperburst and these would be extremely rare events as the build up of the exploding layer requires about a millennium of accretion history. Despite its large energy output, the hyperburst did not produce, due to its depth, any noticeable increase in luminosity during the accretion phase and is only identifiable by its imprint on the later cooling of the neutron star.