Superbursts: Nuclear Fury on Neutron Stars

1. Superbursts: Nuclear Fury on Neutron Stars

2. Sources of Thermonuclear Bursts: LMXBs Containing Neutron Stars X-ray binaries near the Galactic center as seen with RXTE/PCA • Accreting neutron stars in low mass X-ray binaries. • Roughly 70 burst sources known. • Concentrated in the Galactic bulge. Credit: C. B. Markwardt

3. “Normal” Thermonuclear Bursts • 10 - 20 s flares. • Thermal spectra which soften with time. • 3 - 12 hr recurrence times, sometimes quasi-periodic. • ~ 1039 ergs • H and He primary fuels 4U 1636-53 Intensity Time (s)

4. First Superburst from 4U 1735-44 (BeppoSAX/WFC) • Long, 3 - 5 hr flares seen to date from 6 low mass X-ray binaries (LMXB). • Spectra consistent with thermal, show softening with time. • Two superbursts from 4U 1636-53, 4.7 yr apart. • 1,000 x more energy than standard Type I bursts. Cornelisse et al. (2000)

5. RXTE and BeppoSAX Observe “Superbursts” from Accreting Neutron Stars • Long, 3-5 hr. X-ray bursts observed from 5 accreting neutron star binaries (Heise et al. 2000; Strohmayer 2000; Wijnands 2001). • Bursts reveal new regime of nuclear burning. 1,000 times more energy release than normal bursts. • Stories made headlines in national papers, Washington Post, NY Times.

6. Source Summary

7. RXTE Observes Superbursts from 4U 1636-53 • PCA observations of superburst from 4U 1636-53 (Strohmayer 2002). • ASM detections of two superbursts spaced by ~ 5 years (Wijnands 2001). Only source with recurrence time constraint. • Peak flux ~ 1/2 neutron star Eddington limit. • Release of nuclear energy at greater depth than for normal bursts. 12C burning is the likely energy source. Normal burst Normal bursts Superburst Superburst

8. Superbursts from 4U 1636-53: RXTE/ASM detections See Wijnands (2001)

9. Superbursts observed with RXTE/PCA

10. RXTE Observes Three Hour Thermonuclear Burst from a Neutron Star (4U 1820-30) Strohmayer & Brown (2002) • Burst produced ~ 2 x 1042 ergs in X-rays, perhaps 10 - 20x more energy not seen (neutrinos; heat flowing into the crust). • Energy source likely carbon burning at great depth (~1013 g cm-2).

11. Superburst from 4U 1820-30: Carbon Production • Thermonuclear burning is stabilized at high accretion rates (ie. No bursts). • Lower peak burning temperatures will synthesize lots of Carbon. • Higher temperature during unstable burning yields little Carbon No bursts superburst bursts Strohmayer & Brown (2002)

12. A Carbon “bomb” on a Neutron Star • Too much energy for unstable helium burning • Carbon burning can supply total energy, recurrence time ~ 10 years. • Carbon ignites at 1013 g cm-2. Total energy is ~ 10 - 20 times greater than X-ray fluence. • Significant energy loss to neutrinos, energy will flow inward to be released on longer timescale. 12C Ignition curve Strohmayer & Brown (2002) Strohmayer & Brown (2002)

13. Carbon Flashes on Neutron Stars Cumming & Bildsten 2001

14. Carbon Flashes on Neutron Stars • Superburst properties strongly tied to detailed nuclear physics. • Photodissociation of heavy rp process ashes can provide most of superburst energies. Schatz, Bildsten & Cumming 2003

15. Superburst from 4U 1820-30: Spectral Evolution Strohmayer & Brown (2001) • Peak flux consistent with Eddington limit from neutron star. • Broad ~ 6 keV line and ~ 9 keV edge from reflection off inner disk. • New probes of disks and neutron star.

16. Superburst from 4U 1820-30: Spectral Modelling • Long decay timescale gives high signal to noise spectra. • Thermal (black body) spectrum strongly preferred for continuum. • Discrete components, ~ 6 keV broad line, and 9 keV edge required. RXTE/PCA spectrum RXTE/PCA spectrum

17. Superburst from 4U 1820-30: Disk Reflection • Discrete spectral components likely due to reflection of burst flux from disk. • Broad Fe Ka line and smeared edge. • Line and edge parameters vary significantly through burst. • Broad Fe line gives evidence for relativistic disk. Strohmayer & Brown (2002)

18. Probing the Accretion disk in 4U 1820-30: Reflection Spectra • Fitting disk reflection models (with David Ballantyne, CITA). • Thermal (black body) spectrum reflected from slab geometry • relativistic blurring included. Model gives good fits. • We see changes in the ionization state and inner edge location. Preliminary evidence for Fe abundance changes.

19. Oscillations at Burst Onset Intensity An X-ray burst from 4U 1636-53 with 1.7 ms oscillations. Time 4U 1636-53

20. Evidence for Rotational Modulation Surface Area Time Strohmayer, Zhang & Swank (1997) Spreading hot spot. Intensity • Oscillations caused by hot spot on rotating neutron star • Modulation amplitude drops as spot grows. • Spectra track increasing size of X-ray emitting area on star.

21. Bursts from the Accreting ms Pulsar XTE J1814-338 • Oscillations in bursts from J1814 are at the known spin frequency (measured during the persistent emission). • Along with SAX J1808, confirms spin modulation of burst oscillations.

22. RXTE/PCA Observes Superburst from 4U 1636-53 • Unique double precursor near rise of the burst. • Heat flow from below may trigger H/He layer.

23. Pulsations During the Superburst from 4U 1636-53

24. Superburst Pulsations in 4U 1636-53 • Pulse train lasts 900 seconds. Much longer than in normal (short) bursts. • Frequency drifts by about 0.03 Hz in 800 s. Much smaller than drift in normal bursts. • Consistent with orbital modulation of neutron star spin frequency. • XRT in fast timing mode could reach interesting limits.

25. Predicted Orbital Modulation from Optical Ephemeris for 4U 1636-53 pulsation interval • 3.8 hr orbital period. Ephemeris from Augusteijn et al. (1998) and Giles et al. (2002). • Only assumption, optical maximum corresponds to superior conjunction of the optical secondary.

26. Phase Coherent Timing with Circular Orbit Model • Circular orbit describes the frequency and phase evolution very well. • Range of parameters allowed due to short data segment. • Coherence ~ 450,000 !

27. Pulse Profile and Phase Residuals • 1% pulse amplitude (mean) • ~30 msec (rms)

28. Implications for the Component Masses in 4U 1636-53 • Velocity constraint (statistical) from 90 to 175 km s-1 • for MS mass - radius relation, donor would be about 0.36 Msun • Favors a more massive secondary than naïve estimates would suggest • However, short data interval and modest phase jitter, could bias the fit.

29. Swift/BAT Response to Superbursts • Superbursts are bright, Lx ~ 1 - 3 x 1038 ergs s-1 (approximately Eddington limt for NS. • 2 - 3 keV thermal spectra. Important to keep low energy response (15 keV acceptable). • Using estimated BAT response matrix ~ 100 - 600 cts s-1 at peak. • Should be easily detected in 128 - 256 s integration. Use Long - Soft trigger. More detailed simulations needed.

30. Superbursts: SWIFT Observations with High Spectral Resolution • XRT observations could probe accretion physics, disk ionization structure, and GR effects if line shape resolved. • Easier reflection modelling, illuminating geometry is known. • Simultaneous optical (UVOT) - X-ray can be used to probe system geometry. • Spectra can be used to probe NS structure.