Hydrogen‐triggered Type I X‐Ray Bursts in a Two‐Zone Model

We use the two-zone model of Cooper & Narayan to study the onset and timeevolution of hydrogen-triggered type I X-ray bursts on accreting neutron stars.At the lowest accretion rates, thermally unstable hydrogen burning igniteshelium as well and produces a mixed hydrogen and helium burst. For somewhathigher accretion rates, thermally unstable hydrogen burning does not ignitehelium and thus triggers only a weak hydrogen flash. The peak luminosities ofweak hydrogen flashes are typically much lower than the accretion luminosity.These results are in accord with previous theoretical work. We find that aseries of weak hydrogen flashes generates a massive layer of helium thateventually ignites in an energetic pure helium flash. Although previouslyconjectured, this is the first time such bursting behavior has been actuallydemonstrated in a theoretical model. For yet higher accretion rates, hydrogenburning is thermally stable and thus steadily generates a layer of helium thatultimately ignites in a pure helium flash. We find that, for a narrow range ofaccretion rates between the mixed hydrogen and helium burst and weak hydrogenflash regimes, unstable hydrogen burning ignites helium only after a shortseries of weak hydrogen flashes has generated a sufficiently deep layer ofhelium. These bursts have fluences that are intermediate between those ofnormal mixed hydrogen and helium bursts and energetic pure helium flashes.Comment: 9 pages, 7 figures, accepted by Ap