Models of Type I X-ray Bursts from Gs 1826-24: a Probe of Rp-process Hydrogen Burning

The X-ray burster GS 1826-24 shows extremely regular Type I X-ray bursts whose energetics and recurrence times agree well with thermonuclear ignition models. We present calculations of sequences of burst lightcurves using multizone models which follow the nucleosynthesis (αp and rp-processes) with an extensive nuclear reaction network. The theoretical and observed burst lightcurves show remarkable agreement. The models naturally explain the slow rise (duration ≈ 5 s) and long tails (≈ 100 s) of these bursts, as well as their dependence on mass accretion rate. This comparison provides further evidence for solar metallicity in the accreted material in this source, and gives a distance to the source of (6.07± 0.18) kpc ξ b , where ξb is the burst emission anisotropy factor. The main difference is that the observed lightcurves do not show the distinct two-stage rise of the models. This may reflect the time for burning to spread over the stellar surface, or may indicate that our treatment of heat transport or nuclear physics needs to be revised. The trends in burst properties with accretion rate are well-reproduced by our spherically symmetric models which include chemical and thermal inertia from the ashes of previous bursts. Changes in the covering fraction of the accreted fuel are not required. Subject headings: accretion, accretion disks — stars: individual (Ginga 1826-238, GS 1826-24) — stars: neutron — X-rays: bursts