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

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.

Accreting neutron stars: heating of the upper layers of the inner crust

“…By confronting the observational data with cooling models one may try to constrain the key parameter of our theory, the pressure Poi, along with other microscopic parameters of neutron star crust. Note, that all up-to-date cooling models available in the literature (e.g., Shternin et al 2007;Brown & Cumming 2009;Page & Reddy 2012;Deibel et al 2015;Wijnands et al 2017;Brown et al 2018;Meisel et al 2018;Potekhin & Chabrier 2021;Parikh et al 2021;Page et al 2022;Mendes et al 2022) are based on the results of the traditional approach. This means that the constraints on the properties of superdense matter obtained from these models should be revisited, and this can be done by employing the nHD models of the accreted crust developed in the present paper.…”

“…The most intriguing result of subsequent works was the conclusion that deep crustal heating is insufficient for explaining the thermal relaxation of the crust for many observed sources. In order to explain the observations, a number of authors were forced to include phenomenological shallow heating sources in their simulations (Brown & Cumming 2009;Deibel et al 2015;Waterhouse et al 2016;Parikh et al 2018Parikh et al , 2019Page et al 2022;Lu et al 2022). Unfortunately, the nature of these sources remains largely unidentified.…”

Crust Cooling of Soft X-Ray Transients—the Uncertainties of Shallow Heating

“…The upper limits still cannot explain the quiescent light curve of Aql X-1 and MAXI J0556-332 which need a large amount of shallow heat. It is worth mentioning that recently Page et al (2022) proposed a new "hyperburst" in the MAXI J0556-332 neutron star before the end of its first outburst, and such a deep explosion could provide an excellent fit to the cooling data. However, it is difficult to witness another one in SXTs.…”