Millisecond Oscillations in Thermonuclear X-ray Bursts

1. Millisecond Oscillations in Thermonuclear X-ray Bursts Michael Muno (UCLA/Hubble Fellow)

2. Outline • Introduction to Thermonuclear X-ray Bursts • Origin of the Oscillations • Questions about the Oscillations • Models for the Oscillations • Key Observations with RXTE and Beyond

4. An X-ray Burst Occurs Burst rise times, decay times, energetics, and recurrence times are all consistent with models of unstable He burning on a neutron star.

5. Burst Oscillations Unstable He burning is not likely to be uniform, so as the neutron star rotates, variations in its surface brightness should produce oscillations in the X-ray flux.

6. Burst Oscillations: Basics • Detected from 12 of ~65 burst sources • Frequencies characteristic to each source • Distributed uniformly between 270-620 Hz

8. Signs of Neutron Star Rotation • Two sources are persistent pulsars Chakrabarty et al. (2003); Strohmayer et al. (2003)

9. Signs of Neutron Star Rotation • Two sources are persistent pulsars • Oscillations are nearly coherent Strohmayer & Markwardt (2002)

10. Signs of Neutron Star Rotation • Two sources are persistent pulsars • Oscillations are nearly coherent • Frequencies are stable to a few parts in 1000 Strohmayer et al. (1998); Strohmayer & Markwardt (1999); Giles et al. (2002); Muno, Chakrabarty, Galloway, & Psaltis (2002a)

11. Signs of Neutron Star Rotation • Two sources are persistent pulsars • Oscillations are nearly coherent • Frequencies are stable to a few parts in 1000 • Strong in rise of bursts Strohmayer et al. 1998

12. Signs of Neutron Star Rotation • Two sources are persistent pulsars • Oscillations are nearly coherent • Frequencies are stable to a few parts in 1000 • Strong in rise of bursts • Amplitudes consistent with small temperature contrast Muno, Özel, & Chakrabarty 2003

13. What is There to Explain?

14. What is There to Explain? • The persistence of the oscillations rules out simple inhomogeneities in the burning. • The upward drift in frequency suggests that the brightness pattern moves opposite the sense of the rotation, such that :

15. Models for the Oscillations Cumming & Bildsten 2000

16. Models for the Oscillations Cumming & Bildsten 2000 • The burning layer expands at the start of a burst, and, because angular momentum is conserved, it slows down. As the burning layer re-couples, it speeds up (Strohmayer et al. 1997; Cumming & Bildsten 2000). • Frequency drift expected is not quite large enough (Cumming et al. 2002).

18. Models for the Oscillations • The pressure gradient across the cooling front can drive a zonal flow around the neutron star, which in turn could carry vortices (Spitkovsky, Levin, & Ushomirsky 2003).

19. Models for the Oscillations • Rossby modes may be excited in the surface layers of the neutron star, which could lead to periodic temperature variations close to the equator (Heyl 2003; Lee 2003).

20. Key Observations Phase connection: • Fold data in short (0.25 s) intervals about a trial phase model. • Measure phases of each folded profile. • Fit phase residuals in to derive corrections to the initial model. • Iterate until phase residuals are consistent with zero.

21. Instability in the Oscillations

22. Instability in the Oscillations • 20% of the oscillations do not evolve smoothly in phase. This indicates that there are: • Phase jumps of ~0.1 cycle, • Sudden frequency changes (0.25 Hz in 0.25 s, or • Signals present simultaneously at two frequencies. Muno, Chakrabarty, Galloway, & Psaltis 2002a; see also Miller 2000; Strohmayer 2001.

23. Signs of More Surface Modes? We have observed: • Simultaneous signals separated by ~1 Hz in two cases. • Sidebands with frequencies 30 Hz below the main signal from 4U 1728-34. Miller 2000; Galloway et al. 2000; Muno et al. 2002a.

24. Profiles of the Oscillations Wherever we define a frequency model, we know the phase as a function of time and can fold the data coherently. • Average amplitudes are ~5% rms. • Upper limits on harmonic and sub-harmonic signals are <2%. (Muno, Özel, & Chakrabarty 2002b)

25. Profiles of the Oscillations

26. Upper Limits on Harmonic Content Muno, Özel, & Chakrabarty 2002b Harmonic amplitudes are less than 5% of the fundamental amplitudes.

27. Upper Limits to Harmonics Muno, Özel, & Chakrabarty 2002b

28. Constraints from Lack of Harmonics Muno, Özel, & Chakrabarty 2002b • Requires either that the brightness pattern is symmetric on the neutron star, or • That the signal from the star is attenuated by a scattering corona.

29. Beyond RXTE • A future timing mission could detect: • Oscillations from a larger sample of sources • Signals from multiple surface modes • Harmonic content • In the meantime, theory could address • Expected frequencies and strengths of surface modes • Whether scattering can explain the burst spectra, lack of harmonics, and energy dependence of oscillations.

31. Detecting More Oscillations • When oscillations are not detected, the upper limits on their frac-tional amplitudes are only a factor of a few below those of the detections.