Neutrino Losses in Type I Thermonuclear X-Ray Bursts: An Improved Nuclear Energy Generation Approximation
Abstract
Type I X-ray bursts are thermonuclear explosions on the surface of accreting neutron stars. Hydrogen rich X-ray bursts burn protons far from the line of stability and can release energy in the form of neutrinos from β-decays. We have estimated, for the first time, the neutrino fluxes of Type I bursts for a range of initial conditions based on the predictions of a 1D implicit hydrodynamics code, Kepler, which calculates the complete nuclear reaction network. We find that neutrino losses are between 6.7× {10}-5 and 0.14 of the total energy per nucleon, {Q}nuc}, depending upon the hydrogen fraction in the fuel. These values are significantly below the 35% value for neutrino losses often adopted in recent literature for the rp-process. The discrepancy arises because it is only at β-decays that ≈ 35 % of energy is lost due to neutrino emission, whereas there are no neutrino losses in (p,γ ) and (α ,p) reactions. Using the total measured burst energies from Kepler for a range of initial conditions, we have determined an approximation formula for the total energy per nucleon released during an X-ray burst, {Q}nuc} =(1.31+6.95 \overline{X}-1.92 {\overline{X}}2) {MeV} {nucleon}}-1, where \overline{X} is the average hydrogen mass fraction of the ignition column, with an rms error of 0.052 {MeV} {nucleon}}-1. We provide a detailed analysis of the nuclear energy output of a burst and find an incomplete extraction of mass excess in the burst fuel, with 14% of the mass excess in the fuel not being extracted.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- January 2019
- DOI:
- 10.3847/1538-4357/aaeed2
- arXiv:
- arXiv:1808.02225
- Bibcode:
- 2019ApJ...870...64G
- Keywords:
-
- methods: numerical;
- stars: neutron;
- X-rays: bursts;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- 8 pages, 5 figures, published in ApJ