Sensitivity of Nuclear Reaction Rates in X-ray Burst Models

TL;DR

The paper tackles the challenge of nuclear reaction-rate uncertainties in Type I X-ray bursts by performing a large-scale sensitivity study across a 32-system grid spanning $X_{\mathrm{H,acc}}$, $Z$, and $\dot m/\dot m_{\mathrm{Edd}}$. It implements a two-stage framework that couples a semi-analytic ignition model (SETTLE) with post-processing nucleosynthesis (NucNet Tools) and a time-dependent single-zone network (ONEZONE) to evaluate $2{,}708$ rate variations using two sensitivity metrics, $F_{\mathrm{lc}}$ and $F_{\mathrm{ash}}$, on a baseline REACLIB v2.2 network. The study identifies 41 reactions that significantly modify burst light curves and 187 that alter final ashes, with bottlenecks in the $\alpha p$- and rp-process paths largely governing the observables and a marked dependence on ignition composition $X_{\mathrm{H,ign}}$. Notably, He-rich bursts can elevate $^{12}$C yields to about $18\%$, close to the $20\%$ threshold proposed for carbon superburst ignition, highlighting key reactions for experimental focus and underscoring the importance of reducing nuclear-physics uncertainties in XRB models for robust interpretation of observations and superburst scenarios.

Abstract

Type I X-ray bursts (XRBs) are thermonuclear runaways on the surface of accreting neutron stars, powered by rapid proton-capture and alpha-capture processes on neutron-deficient nuclei. Uncertainties in the corresponding reaction rates remain a major limitation in modeling burst light curves and ashes. We present a systematic study of the sensitivity of XRB models to uncertainties in charged-particle-induced reaction rates across a broad parameter space of accretion rates and fuel compositions in low-mass X-ray binaries. The study proceeds in two stages: ignition conditions are first determined with a semi-analytic framework coupled to a full reaction network, followed by a sensitivity analysis using the ONEZONE model with individual rate variations. We identify 41 reactions that alter the burst light curve and 187 that significantly impact final abundances. Reactions on bottleneck isotopes in the alpha-p- and rp-process paths strongly affect both observables, while most (p, gamma) reactions on medium-mass (A > 32) and heavy-mass (A > 55) nuclei influence only the final composition. Medium-mass cases dominate in He-rich bursts, where the reaction flow terminates earlier, while heavy-mass cases appear in mixed H and He bursts with extended rp-process paths reaching A ~ 110. We identify a subset of reactions whose rate uncertainties exert influence on the final 12C yield in helium-rich bursts, which could have important consequences for the mechanism of ignition of carbon superbursts. Our results identify key targets for nuclear reaction experiments to reduce nuclear physics uncertainties in XRB models.

Figures (10)

• Figure 1: Temperature and density profiles of the accreted envelope as functions of column depth. (a) Temperature profiles for fixed hydrogen mass fraction ($X_{\rm H,acc}=0.5$) while varying accretion rate ($\dot m$) and metallicity ($Z$). (b) Temperature profiles at fixed accretion rate ($\dot m = 0.2,\dot m_{\rm Edd}$) for different $X_{\rm H,acc}$ and $Z$. (c) Corresponding density profiles for the same cases shown in panel (a). (d) Corresponding density profiles for the cases in panel (b). Refer to Table \ref{['tab:ignition_data']} for the system acronym and parameter space values. The square and circles indicate, for each model, the ignition depth calculated by the SETTLE code and with ONEZONE following the process described in the text.
• Figure 2: Nuclear composition of the accreted material at the neutron star surface (blue line) and at ignition depth (orange line), after the settling phase simulated using the NucNet Tools single-zone reaction network code. The lower panel in each sub-figure shows the abundance ratio in each isotopic abundance between ignition and accretion depth. Binary system parameters are as follows: (a) $X_\mathrm{H,acc}= 0.7$, Z=0.02, and $\dot m=0.1 \dot m_\mathrm{Edd}$. (b) $X_\mathrm{H,acc}= 0.7$, Z=0.01, and $\dot m=0.1 \dot m_\mathrm{Edd}$ (c) $X_\mathrm{H,acc}= 0.7$, Z=0.02, and $\dot m=0.5 \dot m_\mathrm{Edd}$ (d) $X_\mathrm{H,acc}= 0.3$, Z=0.02, and $\dot m=0.1 \dot m_\mathrm{Edd}$
• Figure 3: (a) Hydrogen mass fraction at ignition ($X_{H,\rm ign}$) as a function of accreted hydrogen fraction ($X_{H,\rm acc}$) for various accretion rates ($\dot m$) and two metallicities. Circular markers indicate $Z = 0.02$, while square markers indicate $Z = 0.01$. Different colors correspond to different values of $\dot m / \dot m_{\rm Edd}$. (b) Ignition column depth vs hydrogen mass fraction at ignition for two metallicities.
• Figure 4: Baseline X-ray burst light curves for 32 systems. Panels are arranged so that $X_{H,\rm ign}$ decreases from the bottom-right toward the top-left, with the top row representing the most helium-rich cases. The color bar indicates the amount of hydrogen mass fraction at ignition ($X_{H,\rm ign}$).
• Figure 5: Baseline isotopic abundance distributions for the same 32 systems, using the identical panel ordering as Figure \ref{['fig:dasfigurinene_lc']}.