# Presentation: 871818

## Superbursts: Nuclear Fury on Neutron Stars

## 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

## “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

- Time (s)

- 4U 1636-53

- Intensity

## First Superburst from 4U 1735-44 (BeppoSAX/WFC) 

- Cornelisse et al. (2000)

- 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.

## 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.

## Source Summary

## RXTE Observes Superbursts from 4U 1636-53

- Superburst

- Normal bursts

- 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.

- Superburst

- Normal burst

## Superbursts from 4U 1636-53: RXTE/ASM detections 

- See Wijnands (2001)

## Superbursts observed with RXTE/PCA 

## RXTE Observes Three Hour Thermonuclear Burst from a Neutron Star (4U 1820-30) 

- 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).** **

- Strohmayer & Brown (2002)

## Superburst from 4U 1820-30: Carbon Production 

- Strohmayer & Brown (2002)

- bursts

- No bursts

- superburst

- 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

## 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)

## Carbon Flashes on Neutron Stars  

- Cumming & Bildsten 2001

## Carbon Flashes on Neutron Stars  

- Schatz, Bildsten & Cumming 2003

- Superburst properties strongly tied to detailed nuclear physics.
- Photodissociation of heavy rp process ashes can provide most of superburst energies.

## Superburst from 4U 1820-30: Spectral Evolution

- 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.

- Strohmayer & Brown (2001)

## 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

## Superburst from 4U 1820-30: Disk Reflection 

- Discrete spectral components likely due to reflection of burst flux from disk.
- Broad Fe K line and smeared edge.
- Line and edge parameters vary significantly through burst.
- Broad Fe line gives evidence for relativistic disk.

- Strohmayer & Brown (2002)

## 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.

## Oscillations at Burst Onset

- An X-ray burst from 4U 1636-53 with 1.7 ms oscillations.

- Time

- Intensity

- 4U 1636-53

## Evidence for Rotational Modulation

- 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.

- Surface Area

- Time

- Intensity

- Spreading hot spot.

- Strohmayer, Zhang & Swank (1997)

## 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.

## RXTE/PCA Observes Superburst from 4U 1636-53

**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.

## Pulsations During the Superburst from 4U 1636-53

## 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.

## Predicted Orbital Modulation from Optical Ephemeris for 4U 1636-53  

- 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.

- pulsation interval

## 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 !

## Pulse Profile and Phase Residuals

- 1% pulse amplitude (mean)
- ~30 sec (rms)

## 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.

- 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.

**Swift/BAT Response to Superbursts**

## 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.