2003ApJ...595.1077C

I present ignition models for type I X-ray bursts and superbursts from the ultracompact binary 4U 1820-30. A pure helium secondary is usually assumed for this system (which has an orbital period ~11 minutes); however, some evolutionary models predict a small amount of hydrogen in the accreted material (mass fraction X∼0.1). I show that the presence of hydrogen significantly affects the type I burst recurrence time if X≳0.03 and the CNO mass fraction ≳3x10^–3. When regularly bursting, the predicted burst properties agree well with observations. The observed 2-4 hr recurrence times are reproduced for a pure He companion if the time-averaged accretion rate is <M>{dot}~7-10x10^–9M_☉yr^–1 or a hydrogen-poor companion if <M>{dot}~4-6x10^–9M_☉yr^–1. This result provides a new constraint on evolutionary models. The burst energetics are consistent with complete burning and spreading of the accreted fuel over the whole stellar surface. Models with hydrogen predict ∼10% variations in burst fluence with recurrence time, which could perhaps distinguish the different evolutionary scenarios. I use the accretion rates determined by matching the type I burst recurrence times to predict superburst properties. The expected recurrence times are ~1-2 yr for pure He accretion (much less than that found by Strohmayer & Brown) and ~5-10 yr if hydrogen is present. Determination of the superburst recurrence time would strongly constrain the local accretion rate and thermal structure of the neutron star.