Fast X-ray Oscillations During Magnetar Flares

1. Fast X-ray Oscillations During Magnetar Flares • Three events to date: • March 5th 1979: SGR 0526-66 • August 27th 1998: SGR 1900+14 • December 27th 2004: SGR 1806-20 • Powered by global magnetic instability (reconfiguration), crust fracturing. Hurley et al. (1998): Ulysses • Short, hard, luminous initial pulse. • Softer X-ray tail persists for minutes, pulsed at neutron star spin period. • Emission from a magnetically confined plasma. • 1015 G magnetic fields implied (Thompson & Duncan 95)

2. NASA’s Rossi X-ray Timing Explorer (RXTE) Launched in December, 1995, in orbit for 11.5 yr! 12th Observing cycle currently underway http://heasarc.gsfc.nasa.gov/docs/xte/xte_1st.html • RXTE’s Unique Strengths • Large collecting area • High time resolution • High telemetry capacity • Flexible observing

3. SGR 1806-20, 2004 December Giant Flare Israel et al. (2005)

4. Oscillations in the 2004 December SGR 1806-20 Flare • RXTE recorded the intense flux through detector shielding. • Israel et al. (2005) reported a 92 Hz quasi-periodic oscillation (QPO) during a portion of the flare. • Oscillation is transient, or at least, the amplitude time dependent, associated with particular rotational phase, and increased un-pulsed emission. • Also evidence presented for lower frequency signals; 18 and  30 Hz. • Suggested torsional vibrations of the neutron star crust. Israel et al. 2005

5. A Look Back at the SGR 1900+14, 1998 August Flare • SGR 1900+14 flare was observed in a manner very similar to the SGR 1806 flare. • Same time resolution, but non-optimal read-out time resulted in data gaps. • Initially searched each good data interval as a whole. • 84 Hz QPO detected in 4th data interval searched. The first after the pulse structure had re-emerged. • Strohmayer & Watts (2005)

6. SGR 1900+14: 84 Hz signal • 84 Hz signal localized in time (rotational phase). • ~20 % (rms) amplitude • Not centered on a pulse peak. No other impulsive signals found, but what about weaker, persistent modulations?

7. SGR 1900+14: Other QPO Signals • Computed average power spectra centered on the rotational phase of the 84 Hz QPO. • A sequence of frequencies was detected: 28, 53.5, and 155 Hz! • Amplitudes in the 7 – 11% range. • Strong phase dependence: no signals detected from phases adjacent phase regions. • 4 frequencies in SGR 1900+14, a sequence of toroidal modes?

8. SGR 1806-20: RHESSI Confirmation of the Oscillations • Ramaty High Energy Solar Spectroscopic Imager (RHESSI) also detected the December, 2004 flare from SGR 1806-20 (Hurley et al. 2005). • Timing study by Watts & Strohmayer (2006) confirms 92 Hz oscillation, and evidence for higher frequency (626 Hz) modulation.

9. SGR 1806-20: RHESSI Confirmation of the Oscillations • Ramaty High Energy Solar Spectroscopic Imager (RHESSI) also detected the December, 2004 flare from SGR 1806-20 (Hurley et al. 2005). • Timing study by Watts & Strohmayer (2006) reveals evidence for much higher frequency oscillation in SGR 1806-20 flare, at 625 Hz.

10. The SGR 1806-20 Flare Re-visited • Data now public. • Phase averaging of power spectra confirms strong 92 Hz QPO. • Used Phase averaging analysis as in SGR 1900+14 hyperflare. Detect several new frequencies.

11. The SGR 1806-20 Flare Re-visited II • Strong detection of 625 Hz signal in latter portions of the flare. • Evidence for higher frequencies too! Phase averaging reveals a 148 Hz QPO feature with high significance, but lower amplitude.

12. The SGR 1806-20 Flare Re-visited: Additional Oscillation Frequencies • SGR 1806 flare data now public. • Phase averaging of power spectra confirms strong 92 Hz QPO. Phase averaging also shows 625 Hz oscillation with high significance. Detected 200 – 250 s after initial spike.

13. Neutron Star Crusts: A Brief History • Existence of a crust in a “normal” neutron star is not “controversial.” Ruderman (1968) suggested that radio pulsations from the first pulsars were due to torsional vibrations of crust. • To good approximation crust is a Coulomb solid, that solidifies at G = (Ze)2 / akT> 175. • Implies shear modulus, m, scales as n(Ze)2 / a • Ogata & Ichimaru (1990), Strohmayer et al. (1991) calculated m and explored oscillation mode implications. • Crust properties also linked with other observables, ie. Glitches and spin-down of pulsars, for example. Piro (2005)

14. Shear Waves Credit: Larry Braile, Purdue Univ.

15. SGR 1900+14: Toroidal (torsional) Oscillation Modes • A neutron star crust supports shear (toroidal) modes. Purely transverse motions. Modes studied theoretically (McDermott, Van Horn & Hansen (1988), Schumaker & Thorne (1983), Duncan (1998), Strohmayer et al. (1991), Samuelsson & Andersson (2007). • Angular dependence of modes described by spherical harmonic functions (l, m); and a radial eigenfunction (n), l tn • l gives the total number of nodal planes, and m the number of azimuthal planes. An l=7, m=4 toroidal mode (Anna Watts) (1.71 - 0.71M1.4/R10)1/2 n(2t0) = 29.8 (R10)-1 Hz (0.87 - 0.13M1.4/R102) n(lt0) = n(2t0)[ l(l+1)/6 ]1/2 [1 + (B/Bm)2 ]-1/2

16. The SGR 1806-20 Flare: thickness of the crust Strohmayer & Watts (2006) • For n = 0 modes, effective wavelength is R, for n > 0 it is DR, if Vs = constant, then fn=0 / fn>0 ~ DR/R. • Frequencies at 625 Hz and higher are likely n > 0 modes. Detection of n = 0 and n = 1 constrains crust thickness!

17. Torsional modes: shear modulus, magnetic fields, and mode periods • 30 – 150 Hz frequencies are consistent with current estimates of n = 0 mode frequencies, with n > 0 modes above 600 Hz. Piro (2005) Piro (2005) Speed of shear waves, Vs, set by the shear modulus. Vs ~ const (1,000 km/sec) not too bad an approximation.

18. Mode Excitation, the Earth Analogy • Crust fracture in general will excite global modes. • Many such modes observed in the days after the 2004 December Sumatra – Andaman great earthquake. • Spectrum of modes excited depends on fracture geometry, but non-trivial patterns possible. Park et al. (2005)

19. Implications for Neutron Star Structure • Recently, Samuelsson & Andersson (2007) computed torsional modes in full GR (Cowling approximation). • Determine “allowed” regions in M - R plane that can match observe mode sequences in SGRs 1806-20 and 1900+14. • Caveat: No magnetic field corrections at all.

20. Implications for Neutron Star Structure Frequencies (Hz) 29.0 : 2t0 92.7 : 6t0 150.3 : 10t0 625.5 : 1t1 : 2t1 1837 : 1t4 APR (A18+UIX+v) Sahu, Basu & Datta (1993) Determine “allowed” regions in M - R (B-field) plane that can match observed frequencies. Calculations in progress for a range of realistic EOS (Schwarz & Strohmayer 2008, in preparation).

21. Constraints on Quark Stars? Watts & Reddy (2007) • Watts & Reddy (2007) investigated crusts on strange quark stars. • Computed torsional mode periods for thin nuclear crusts, and also quark nugget crusts. In general, such stars have thinner crusts. • Computed modes very difficult to reconcile with observed periods.

22. Time and Frequency Variations • 90 Hz QPO shows rather complex temporal, phase, and frequency variations. • Amplitudes not constant in time (episodic). • Several factors could be at work; changes in field and particle distributions. Energy exchange with the core. Strohmayer & Watts 2006

23. Persistence of QPOs

24. Theoretical Issues • Recognized that magnetic coupling of crust with core will likely be significant. Need “global” mode calculations (Levin 2006; Glampedakis et al. 2006; Sotani et al. 2006). • Levin (2006) initially argued that torsional modes will radiate Alfven waves into the core and damp too quickly to be observed (~ 1 sec). • Feedback, energy exchange between crust and core (Levin 2007). • Excitation of modes in the crust, analogies with earthquake fractures. • We see modulations in the X-ray flux, can these be produced by crust motions. • Signal amplification, perhaps modest amplitudes are visible (beaming?)

25. Crust - Core coupling • Levin argues that energy exchanges between the crust and a continuum of MHD modes in the core. • Solving initial value problem, finds QPOs excited at the so called “turning points” or edges of the MHD continuum. Lowest frequency modes (18, 25 Hz). • Also finds drifting QPOs, and amplification of these features near pure crust mode frequencies. Levin (2007)

26. X-ray Modulation Mechanism Timokhin, Eichler & Lyubarsky (astro-ph/07063698) • Suggest that modulation of the particle number density in the magnetosphere by torsional motion of the crust produces the oscillations. • Estimate that 1% amplitude of crust motion needed to explain the observed QPO amplitudes. • The angular dependence of the optical depth to resonant Compton scattering may account for phase dependence.

27. Conclusions, and many questions! • Detection of multiple frequencies with consistent l-scaling, and frequencies consistent with n>0 modes is highly suggestive of torsional oscillations, but need more data! • Two (three, March 5th?) out of three with similar phenomena, suggests fundamental property of magnetars. • Further understanding and detections could help constrain neutron star properties (EOS, and crust properties, magnetic fields). Unfortunately, giant flares are rare! • More theory needed: new mode calculations (with magnetic fields, etc.) • How are modes excited and damped? How do the mechanical motions modulate the X-ray flux? • Could magnetic mode splitting be observed, constrain field geometry?

28. Inside Neutron Stars Superfluid neutrons ??? Pions, kaons, hyperons, strange quark matter, quark-gluon plasma? r ~ 1 x 1015 g cm-3 • The physical constituents of neutron star interiors still largely remain a mystery after 35 years.

29. QCD phase diagram: New states of matter Rho 2000, thanks to David Kaplan • Aspects of QCD still largely unconstrained. • Recent theoretical work has explored QCD phase diagram (Alford, Wilczek, Reddy, Rajagopal, et al.) • Exotic states of Quark matter postulated, CFL, color superconducting states. • Neutron star interiors could contain such states. Can we infer its presence??

30. Implications for Neutron Star Structure • For SGR 1900+14, 28, 53.5, 84, and 155 Hz sequence is plausibly consistent with l=2, 4, 7, 13 modes (n=0)! • Mode frequencies depend on stellar mass, radius and magnetic field. • If modes correctly identified, then it places constraints on the stellar structure, though, with some caveats (such as magnetic field effects). SGR 1806 SGR 1900 Strohmayer & Watts (2005)

31. Magnetar hyper-flares: Whole lotta shakin’ goin’ on Tod Strohmayer, NASA’s Goddard Space Flight Center

32. Fundamental Physics: Existence of New States of Matter? • Theoretical work suggests quark matter could exist in neutron stars, possibly co-existing with a nuclear component. • Mass – Radius measurements alone may not be enough to discriminate the presence of quark matter. • Other observables, such as global oscillations might be crucial. Alford et al. (2005)

33. Anatomy of a Hyperflare • Three events to date: • March 5th 1979: SGR 0526-66 • August 27th 1998: SGR 1900+14 • December 27th 2004: SGR 1806-20 • Powered by global magnetic instability (reconfiguration), crust fracturing. Hurley et al. (1998): Ulysses • Short, hard, luminous initial pulse. • Softer X-ray tail persists for minutes, and reveals neutron star spin period. • Emission from a magnetically confined plasma. • 1015 G magnetic fields implied (Thompson & Duncan 95)

34. The Neutron Star Equation of State dP/dr = -r G M(r) / r2 Lattimer & Prakash 2004 • Mass measurements, limits softening of EOS from hyperons, quarks, other exotic stuff. • Radius provides direct information on nuclear interactions (nuclear symmetry energy). • Other observables, such as global oscillations might also be crucial.