Nuclear Physics of X-ray Bursts

Abstract

Thermonuclear X-ray bursts from the surface of accreting neutron stars are the most common astrophysical explosions in our galaxy. They provide a unique window into the physics of neutron stars, the physics of matter under extreme conditions, and the physics of astrophysical thermonuclear explosions. X-ray bursts are powered by a broad range of nuclear reactions that need to be understood to interpret observations. The relevant nuclei are mostly neutron deficient and unstable, and thus experimental information and theoretical understanding is limited and an active area of research in nuclear science. We review the current status of the nuclear physics of X-ray bursts, with special emphasis on new experimental and theoretical information on a large number of reaction rates. As such we provide an overview of the broad experimental and theoretical methods currently used to advance the nuclear physics of X-ray bursts. The new information is used to update the public JINA REACLIB database with 32 new reaction rates based on experimental information, and a new dataset of theoretical statistical model reaction rates where no experimental information is available. Using several models for X-ray bursts that are powered by mixed hydrogen and helium burning, we take advantage of the updated nuclear data to review the current understanding of the nuclear reaction sequences in such X-ray bursts, the modeling of light curves, and predictions of the composition of nuclear ashes.

Figures (23)

• Figure 1: Sources of charged particle reaction rates in the updated JINA REACLIB and used in our X-ray burst model simulations. Red lines mark reactions that were updated in this work using some experimental information as discussed in Sec. \ref{['subsec:individualrate']}. Also shown are the sources of additional rates that are relevant for the main path of an extended rp-process schatzEndPointRp2001a. Black lines mark other reactions in JINA REACLIB where some experimental information has been used in the past to derive a rate and that have not been updated in this work (Sec. \ref{['subsec:nonupdaterate']}). Blue lines mark reactions determined from statistical model calculations, which have also been updated in this work as discussed in Sec. \ref{['subsubsec:HFmodel']}. Violet lines mark reactions previously determined from shell model calculations and included in prior versions of JINA REACLIB as discussed in Sec. \ref{['subsec:shellmodel']}. All reactions that have been identified as important in the sensitivity study of cyburtDependenceXRayBurst2016a are marked as thick lines.
• Figure 2: Ratio of temperature and density dependent $\beta^+$ and electron capture decay half-lives to experimental ground state half-lives. Shown are enhancement ratios for $T=2$ GK, $Y_{\rm e} \rho$=10$^5$ mol/cm$^{-3}$ (black) and $T=0.01$ GK, $Y_{\rm e} \rho$=10$^1$ mol/cm$^{-3}$ (red, dashed) as functions of parent mass number for the main $\beta$-decaying isotopes along the rp-process path.
• Figure 3: $\alpha$-separation energies of Mo isotopes predicted by FRDM2012 (solid line), FRDM1993 (dashed line), AME20 extrapolations (open circles), together with experimental data (solid circles) and a data point for $^{84}$Mo obtained from AME2020 extrapolations for the $^{84}$Mo mass and a new measurement of the $^{80}$Zr mass (red triangle) hamakerPrecisionMassMeasurement2021.
• Figure 4: Nuclear masses used in the present study.
• Figure 5: Updated reaction rates with experimental information that have changed by more then 10% in the temperature range relevant for X-ray bursts (thick lines). Colors represent the maximum ratio between updated rate and previous REACLIB result (factors may indicate increases or decreases): factor 1.1-1.5 (dark blue), factor 1.5-2 (light blue), factor 2-3 (green), factor 3-5 (orange), more than factor of 5 (red). Thin lines illustrate a typical reaction flow during an X-ray burst cyburtDependenceXRayBurst2016a for illustration.