Models of Type I X‐Ray Bursts from 4U 1820−30

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 ≳3 × 10 -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 ⟨ ⟩ ≈ (7–10) × 10 -9 M ☉ yr -1 or a hydrogen-poor companion if ⟨ ⟩ ≈ (4–6) × 10 -9 M ☉ 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.