The physics of Type I X-ray bursts

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Type I X-ray bursts are thermonuclear explosions that occur on accreting neutron stars. Observations and theoretical burst models are discrepant; nearly all models predict bursts occur frequently for all M /M Edd [Special characters omitted.] 1, whereas observations suggest bursts are infrequent when 0.1 [Special characters omitted.] M /M Edd [Special characters omitted.] 0.3 and cease when M /M Edd [Special characters omitted.] 0.3. In this thesis, I address this discrepancy by investigating the physics of ignition and its ramifications. The triple-α reaction produces 12 C, which feeds the hot CNO cycle, and the hot CNO cycle produces 4 He, which feeds the triple-α reaction. This interplay amplifies hot CNO cycle burning and helps thermally stabilize the matter. Ignition requires a breakout from the hot CNO cycle via 15 O(α,γ) 19 Ne, the rate of which is experimentally poorly-constrained. Models in which the rate is lower by a factor ∼ 3 agree with observations, whereas models with higher trial rates, including the fiducial rate, do not. This suggests the true 15 O(α,γ) 19 Ne rate is lower than usually assumed. Superbursts are energetic bursts triggered by 12 C burning. The triple-α reaction generates the 12 C. If present, H immediately fuses with 12 C and destroys the fuel. Stable burning of a solar abundance of accreted matter produces an insufficient amount of 12 C for ignition. If the accreted matter's metallicity is sufficiently above solar, H depletes before 4 He burns and leaves enough 12 C for ignition. 4 He-triggered bursts from systems that exhibit superbursts differ from those that do not: they have shorter durations and are less frequent. Increasing the accreted matter's CNO abundance reproduces both of these observations as well. Astronomers have detected oscillations in burst lightcurves from 20 sources. The rotational modulation of a growing hot spot generates oscillations during the rise, whereas the ε-mechanism drives oscillations during the decay. A long-lived hot spot requires nonequatorial ignition. I show that nonequatorial ignition occurs preferentially at high M , which explains why oscillations occur during the burst rise only when M is high. Furthermore, I show that convection dampens oscillations; this may explain the rarity of oscillation during the burst peak.