Rp-process Nuclosynthesis in Type I X-ray Bursts

1. Rp-process Nuclosynthesis in Type I X-ray Bursts A.M. Amthor Church of Christ, Kingdom of Heaven National Superconducting Cyclotron Laboratory, Michigan State University Department of Physics and Astronomy, Michigan State University Phy 983 - Spring2005

2. Outline • Quick Review of X-ray bursts • Delineation of burst types by total accretion rate • Method of breakout to start the rp-process in a mixed H/He burst • Observations – compared to expectations • Simulations Phy 983 - Spring2005

3. X-ray burst basics – mostly review Burst flux Persistent flux Accretion – of matter from companion star Accumulation – of matter on the NS surface Ignition – near the base of the accreted column Explosion – runaway fusion chain reactions through the ap and rp-process Interesting quantities are: - the total mass accretion rate - the specific accretion rate - the ratio of persistent flux to burst flux - the recurrence time Also the burst duration and regularity Bursts happen for : Phy 983 - Spring2005

4. Burst types For we have which allows unstable CNO H burning. For we have for which the HCNO cycle leads to stable H burning. (Assuming accreted material with ) Burst So for and the burst ignition will be by unstable 3a. Burst Pure He Burst Mixed H/He Burst Phy 983 - Spring2005

5. Breakout to Rp-process (H/He burst) Beyond curve a) is dominant Beyond curve b) is dominant Beyond curve c) the rp-process rate is limited by decays not by By curve d) dominates the flow, then avoiding all decays up to that point 3a flow Hot CNO cycle below curve a) Significant boundaries in temperature vs. density for the development of the rp-process Schatz, Phy 983 notes spring 2003. Phy 983 - Spring2005 Beyond curve a) is dominant

6. Observations Extended study of GS1826-238 Increased total accretion rate for the same type of burst Reduced time to build to critical column depth & Increased temperature in accreted layer from gravitational energy release Reduced recurrence time Line for Measures total accretion rate Phy 983 - Spring2005

7. ms Oscillations From the neutron star binary 4U 1702-429 Oscillations likely caused by asymmetric burst ignition. Frequencies closely related to the neutron star rotation frequencies. Frequency drift possibly caused by expansion of the burning envelope during the burst. Contraction recouples the envelope to the surface resulting in spin up approaching NS’s rotation frequency. Spin up – Spin down ? Burst rise – Burst tail ? Strohmayer, T. E. and L. Bildsten, Compact Stellar X-ray sources, astro-ph/0301544 (2003). Phy 983 - Spring2005

8. Unexplained observations • LMXB with accretion rates consistent with steady bursting which show few or no bursts • Transition between bursting regimes at total accretion rates not consistent with theory • Large frequency drifts in oscillations Phy 983 - Spring2005

9. Simulations Reaction Network Calculations Given adequate hydrogen and slow cooling, burning would continue to a closed cycle in Sn, Sb, and Te. Truncated Network • 1-Zone Model • Constant temperature • Constant density • Limited reaction network van Wormer et al. ApJ. 432:326 (1994) Phy 983 - Spring2005

10. Newer model calculations Still assumes spherical symmetry! • Multi-Zone/1d-Model • Variable temperature • Variable density • 1300 isotopes in adaptive network • Convective and semiconvective mixing and energy transport • Compositional inertia in burst trains Phy 983 - Spring2005

11. Thank you – any questions? ? Phy 983 - Spring2005