Oscillations in Burst Tails

1. Oscillations in Burst Tails Michael Muno (MIT/CSR)

2. Burst Oscillations: Basics • Detected from 12 of ~65 burst sources • Frequencies characteristic to each source • Distributed uniformly between 270 and 620 Hz • Not seen in all bursts from a given source • Seen for up to 15 s • Instantaneously coherent • Drift in frequency by a few Hz • Originate from spin of NS

3. What do we know? • When do burst oscillations occur? • How do their frequencies evolve? • How stable are they? • How do their amplitudes evolve? • What shape are their profiles? • How do they appear as a function of energy?

4. When They Occur

5. Why not in all bursts? The persistent emission. Muno, Remillard, & Chakrabarty 2002; Gierlinski & Done 2002

6. Why not in all bursts? The persistent emission. Muno, Remillard, & Chakrabarty 2002; Gierlinski & Done 2002

7. Why not in all bursts? The persistent emission. Muno, Remillard, & Chakrabarty 2002; Gierlinski & Done 2002

8. Oscillations and the Persistent Emission Muno et al. 2000 Franco 2001; van Straaten et al. 2001

9. Why not in all bursts? The persistent emission.

10. Bursts properties change with the persistent flux. • Peak Flux • Fluence • Duration • Recurrence Time • Presence of Radius Expansion

11. Oscillations, Bursts, and the Persistent Emission Muno et al. 2001; see also Franco 2001; van Straaten et al. 2001

12. Oscillations, Bursts, and the Persistent Emission • Oscillations are only observed when the spectrum is soft and the accretion rate is (presumably) high. • The properties of bursts change differently with spin: • In fast sources (400-600 Hz), bursts get stronger an recur less often as accretion rate increases • In slow (300 Hz), bursts get weaker and recur a bit more often. So perhaps. . . • The spin determines how accretion spreads on the surface, or • A third parameter (e.g., the composition of accreted material) sets the burst properties and the spin of the neutron star.

13. Frequency Evolution

14. Frequency Evolution in Burst Oscillations

15. Frequencies fall in a Narrow Range Muno, Chakrabarty, Galloway, & Psaltis 2002a

16. Frequencies fall in a Narrow Range Muno, Chakrabarty, Galloway, & Psaltis 2002a But see Wijnands et al. (2001)

17. Exceptions Frequency generally increases during a burst, saturating at a nearly constant value. However, out of 68 oscillation trains (as of 2001 September): • Spin-down in 3 bursts (see also Strohmayer et al. 1999)

18. Exceptions Frequency generally increases during a burst, saturating at a nearly constant value. However, out of 68 oscillation trains (as of 2001 September): • Spin-down in 3 bursts (see also Strohmayer et al. 1999) • Simultaneous signals at two frequencies in 2 bursts (also Miller 2000)

19. Frequency Evolution in Burst Oscillations • Oscillations typically drift upwards in frequency by a few Hz. • Frequencies saturate at an approximately constant value. • Drift begins at the start of the burst. • Absolute magnitude of the frequency drift is similar for fast and slow oscillations.

20. Coherence and Stability

21. Models for the Frequency Evolution 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.

22. Distinguishing between Possible Models

23. Distinguishing between Possible Models • Out of 59 oscillation trains: • 37 exhibited evidence for saturation (a non-zero second derivative in frequency) • Exponential models were only favored over polynomials in 6 cases, and only consistent with the data in 15 cases. • The frequencies tend to wander by ~Hz on the time scale of seconds. • (Muno, Chakrabarty, Galloway, & Psaltis 2002a; see also Miller 2000)

24. Instability in the Oscillations

25. Instability in the Oscillations • 12 out of 59 oscillations are not consistent with either exponential or low-order polynomial phase models at the 90% level. 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.

26. Stability of the Asymptotic Frequencies Muno, Chakrabarty, Galloway, & Psaltis 2002a

27. Stability of the Asymptotic Frequencies • Maximum frequencies are stable to about one part in 1000 • Residual dispersion is not consistent with orbital motion in 4U1636-536 Muno, Chakrabarty, Galloway, & Psaltis 2002a; see also Stroh-mayer et al. 1998; Strohmayer & Markwardt 1999; Giles et al. 2002

28. Amplitudes

29. Amplitude Evolution (Muno, Özel, & Chakrabarty 2003)

30. Amplitude Evolution • About 60% of bursts exhibit detectable amplitude variations • Most oscillations exhibit maxima in their amplitudes during the decay of the burst • There are no obvious properties of the burst that can explain these maxima • (Note that the analysis technique is not sensitive to the high-amplitude oscillations during the rise of the burst, because the flux is still low.) (Muno, Özel, & Chakrabarty 2003)

31. Radius Expansion Interrupts Oscillations Muno, Chakrabarty, Galloway, & Psaltis 2002a

32. Profiles

33. 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%.

34. Profiles of the Oscillations

35. Upper Limits on Harmonic Content • Harmonic and sub-harmonic amplitudes are less than 5% of the fundamental in 4U1636-536 and 4U1728-34 • Places constraints on geometry of brightness pattern on neutron star (Muno, Özel, & Chakrabarty 2002b; see talk by Feryal Özel)..

36. Energy Dependence

37. Energy Dependence • The amplitudes of the oscillations increase strongly as a function of energy. The slope of that increase varies from burst-to-burst in any given source. • This behavior is consistent with a hot spot with a temperature contrast of ~0.2 keV. • (Muno, Özel, & Chakrabarty 2003)

38. Energy Dependence • The pulse at high energies appears to lag behind that at low energies. The phase lags vary significantly from burst-to-burst. • This behavior is inconsistent with that expected from Doppler shifts. • (Muno, Özel, & Chakrabarty 2003)

39. What We Know. • Occurrence: Burst oscillations are observed when the persistent spectrum is soft (high accretion rates). The properties of bursts change differently with the persistent flux depending upon the oscillation frequency. • Frequency evolution: Frequencies generally increase by a few Hz and saturate at a nearly constant value. Spin-down and simultaneous signals occur rarely. • Stability: Oscillations are stable only to a part in 1000 on time scales of years, and exhibit 0.1 cycle phase jumps on time scales of <1 s. • Amplitudes: Secondary maxima often appear in the burst tails, without any obvious cause. Oscillations are interrupted by radius expansion. • Profiles: Are sinusoidal to observational accuracy (no harmonics or half-frequency signals in tails of bursts). • Energy dependence: The amplitudes increase strongly with energy. The peak of the profile at 20 keV lags behind that at 3 keV by 150μs.