arXiv subjects

Martin Nava-Callejas

Publications and source records attributed to Martin Nava-Callejas.

7 recordsLinked to original sources

arXiv2026

On the impact of the carbon fusion rate over the properties of superbursts -- Numerical simulations of superbursts with MESA

Context: Superbursts are very energetic explosions in the crust of neutron stars in Low-Mass X-ray Binaries (LMXBs). These are triggered by unstable carbon burning at $T\leq 10^{9}$ K. In recent years, there has been a re-examination of the carbon fusion rate, finding that at these temperatures it might be either smaller or higher with respect to the standard rate from Caughlan \& Fowler (1988). Aims: We explore the consequences changing the carbon fusion rate has over the physics of superbursts. Methods: For simulating superbursts, we employ the public code \mesa\ v.24.08.1, as well as four versions of the carbon fusion reaction rate. Results: A significant enhancement of the reaction rate at $T\leq 10^9$~K would reduce the recurrence and decay times of the superburst, as well as the column depth at ignition. The opposite behavior is observed when the carbon fusion rate is reduced. The maximum temperature reached during the explosion is also sensitive to a change in the carbon fusion rate, leading to either an enhancement or a reduction in the synthesis of $α$-nuclides. These changes are comparable to the effect of reducing the amount of base heating at the bottom of the envelope. Conclusions: The modelling of X-ray superburst is sensitive to the adopted carbon fusion rate at $T\leq 10^{9}$ K. Given its role for determining the ignition conditions, this rate needs to be better experimentally constrained to reduce the uncertainty in nuclear physics when modeling the superburst light curve and the nucleosynthesis of the explosion. At the same time, multizone simulations of superburst also requires improvement in the description of the initially accreted material and stellar conditions prior to the explosion.

astro-ph.HE↗

arXiv2026

On a re-examination of neutron star cooling in transient sources -- No shallow heating required?

Context: For the typical modeling of neutron stars cooling after an accretion episode in Low-Mass X-ray Binaries, an extra heating source of unknown physical origin, \textit{the shallow heating}, is invoked in order to account for the inferred high effective temperatures of the star up to hundreds of days after the end of accretion. The amount of the shallow heating generated in the crust is usually taken to be proportional to the accretion rate, although the proportionality constant may change from source to source. Aims: In this paper, we intend to model the effective temperature data of eight outburst episodes from seven different sources (\mxb, \xte, \exo, \igr, \swift, \rxs\ and \ks) without {\it ad hoc} shallow heating but accounting for the presence of thermonuclear heating due to the burning of the accreted H/He. Methods: We employ the fully relativistic code \texttt{nscool}, which simulates both the crust and core of a neutron star, equipped with a new boundary condition at the envelope/crust transition which considers thermonuclear heating leakage from the envelope into the crust and depends on the mass accretion rate. Results: We find that the neutron star cooling for seven out of eight of these outbursts can be well explained with this new boundary condition and without the requirement of {\it ad hoc} shallow heating. While the qualitative features of \exo\ cooling curve can be explained, a good fitting still requires additional physics.

astro-ph.HE↗

arXiv2025

Post-Supernova Accretion of Light Elements onto a New-Born Neutron Star and NS 1987A

We model early accretion of light elements, He, C, and O, onto a new-born neutron star using the public stellar evolution code MESA, simulating what may happen during the first few years of its life. We find that, under the appropriate conditions, significant amounts of these elements can be accreted up to densities of 10^9 g/cc without triggering a nuclear explosion that would convert them into heavy elements. These results help to understand observations that favor light elements in the atmospheres of young cooling neutron stars, as the one found in the supernova remnant Cassiopeia A, and also add support to the recent indications for the presence of a neutron star, NS 1987A, in the remnant of SN 1987A.

astro-ph.SR↗

arXiv2025

Thermonuclear Heating of Accreting Neutron Stars

We describe a new method to incorporate thermonuclear heating in the envelope of accreting neutron star into long term simulations of their thermal evolution. We obtain boundary conditions for the heat exchange between the envelope and the crust based on stationary models which include nuclear burning and validate these values comparing to the results of the time-dependent code \texttt{MESA}. These simple boundary conditions allow us to explore a large parameter space. We quantify the amount of heat flowing from the envelope into the crust, or viceversa, depending on the mass accretion rate, outburst duration and duty cycle, and especially crust/core physical parameters such as impurities, crustal heating, and neutrino cooling rate.

astro-ph.HE↗

arXiv2024

The effect of opacity on neutron star Type I X-ray burst quenching

One long standing tension between theory and observations of Type I X-ray burst is the accretion rate at which the burst disappear due to stabilization of the nuclear burning that powers them. This is observed to happen at roughly one third of the theoretical expectations. Various solutions have been proposed, the most notable of which is the addition of a yet unknown source of heat in the upper layers of the crust, below the burning envelope. In this paper we ran several simulations using the 1D code MESA to explore the impact of opacity on the threshold mass accretion rate after which the bursts disappear, finding that a higher than expected opacity in the less dense layers near the surface has a stabilizing effect.

astro-ph.HE↗

arXiv2024

Stationary neutron star envelopes at high accretion rates

In this work we model stationary neutron star envelopes at high accretion rates and describe our new code for such studies. As a first step we put special emphasis on the rp-process which results in the synthesis of heavy elements and study in detail how this synthesis depends on the mass accretion rate and the chemical composition of the accreted matter. We show that at very low accretion rate, $\dot{M} \sim 0.01 \dot{M}_{\text{Edd}}$, mostly low mass ($A\leq$ 24) elements are synthesized with a few heavier ones below the $^{40}$Ca bottleneck. However, once $\dot{M}$ is above ${\buildrel \sim \over >} 0.1 \dot{M}_{\text{Edd}}$ this bottleneck is surpassed and nuclei in the iron peak region ($A\sim$ 56) are abundantly produced. At higher mass accretion rates progressively heavier nuclei are generated, reaching $A \sim 70$ at $\dot{M}_{\text{Edd}}$ and $A \sim 90$ at $5 \dot{M}_{\text{Edd}}$. We find that when the rp-process is efficient, the nucleosynthesis it generates is independent of the accreted abundance of CNO elements as these are directly and copiously generated once the $3α$-reaction is operating. We also explore the efficiency of the rp-process under variations of the relative abundances of H and He. Simultaneously, we put special emphasis on the density profiles of the energy generation rate particularly at high density beyond the hydrogen exhaustion point. Our results are of importance for the study of neutron stars in systems in which X-ray bursts are absent but are also of relevance for other systems in describing the low density region, mostly below $10^6$ g cm\mmm, inbetween bursts.

astro-ph.HE↗

arXiv2022

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

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.

astro-ph.HE↗