High-Frequency GW Sources

1. High-Frequency GW Sources Bernard F Schutz Albert Einstein Institute – Max Planck Institute for Gravitational Physics, Golm, Germany and Cardiff University, Cardiff, UK http://www.aei.mpg.de schutz@aei.mpg.de

2. Ground-based GW Astronomy • Existing detectors (cryogenic bars, prototype interferometers, TAMA300) have not seen anything so far. • First-generation interferometric detectors (LIGO, GEO600, VIRGO) will operate soon at sensitivity h ~ 10-21, but may not make detections. • Second-generation detectors (Advanced LIGO, upgraded VIRGO, planned JGWO in Japan?) should reach the sensitivity needed for frequent detections of binary inspiral. • For many potential sources, we cannot even reliably predict sensitivity level needed: pulsars, supernovae, stochastic background. See comprehensive review: Cutler & Thorne, gr-qc/0204090 (GR16 Proceedings). • Focus in this talk on two topics: spinning neutron stars, and sources in the intermediate frequency band (0.1-10 Hz).

3. Spinning Neutron Stars • Continuous-wave (cw) radiation; expect low amplitudes, require long integration times • Many objects with known frequency and position (pulsars), some more with known positions (X-ray sources) • Great interest in detecting radiation: physics of such stars is poorly understood. • After 35 years we still don’t know what makes pulsars pulse. • Interior properties not understood: equation of state, superfluidity, conductivity, solid core, source of magnetic field. • May not even be neutron stars: strange matter!

4. 1s noise in 1-year observation Crab log(h) J1540-6919 J1952+3252 LIGO I J0437-4715 LIGO II J1744-1134 log(f/Hz) Upper limits on some known pulsars

5. How could pulsars radiate? • Crustal asymmetries. Cutler & Thorne: LIGO II will see any with ellipticity e > 2 x 10-7 (fkHz)-2rkpc. Standard NS crust models predict e < 10-5, plausibly much smaller. Likely that young neutron stars are well below spindown limit. • Wobbling neutron stars. If a star is tri-axial, it may precess as it spins(Cutler & Jones). GWs emitted at spin+precession frequency. Effective e < 10-7. • Non-standard stars. If stars have solid cores and/or strange-star equations of state, ellipticities can be larger by factors of perhaps 100. • R-modes. Viscosity from hyperons in core, plus nonlinear effects, seem to overwhelm instability for young stars; not so clear for millisecond pulsars. Strange stars may be strongly unstable. (Owen, Lindblom, Andersson, Kokkotas, …)

6. New mechanism: toriodal B-field flip • Cutler (gr-qc/02060521) adds new twist: Bt has longitudinal tension, squeezing equator inwards, producing prolate crust. This competes with the rotation-induced oblateness, but the crustal strength is low, so it is not hard for Bt to win. • A rigid or elastic prolate body spinning about its long axis will, on a secular timescale, re-orient to spin about a short axis. • Cutler speculates that this can happen even when only the crust is elastic. • Pulsar B-fields not understood, but dynamos require toroidal fields Bt. • When pulsar is formed, strong differential rotation could wind up poloidal field, creating much stronger toroidal component. Near-perfect MHD could sustain this field subsequently. • Bonazzola, Gourgoulhon and collaborators (1995/6) considered gw emission due to distortions created directly by such fields.

7. Natural pulsar model • Cutler’s model leads naturally to geometry where poloidal field is in the spin equator. • Put in numbers, find it can account for entire spindown of millisecond pulsars, and could sustain Wagoner/Bildsten mechanism for LMXB spins. • Caveats: (1) Does not account for all spindown of young pulsars. (2) Need to assume Bp is not perpendicular to Bt (cf Earth field offset angle).

8. Searching for pulsars • LAL library contains codes for making directed and wide-area searches for cw signals. Codes contributed by AEI pulsar group led by M-A Papa, includes A Sintes, S Berukoff, C Aulbert. • FFT-like searches performed by Coherent Demodulation Code, which begins with short-period FFTs (~1 hr, signal modulation not visible), and constructs matched filter demodulation by adding them coherently with appropriate phases. • CDC filters for both phase and amplitude modulation. Uses ephemeris code contributed by Cutler. Key feature: works entirely in narrow frequency band, so is ideal for parallel architectures. Can perform arbitrarily long “FFT”. • Wide-area searches need hierarchical methods. The Hough Transform Code starts with ~1 day demodulated power spectra and does pattern-finding on frequency peaks over ~100 days. • Benchmarks and Grid experiments on teraflop clusters: late 2002.

9. Intermediate-Band Sources • Between ground-based and LISA frequency ranges is the poorly-covered intermediate frequency band, 0.1 Hz – 10 Hz. • A future LISA follow-on mission might target this band because it is relatively clear of “foreground” sources, a good place to look for a cosmological background. • Such a mission would need Sh = 10-48 Hz-1 to reach Wgw = 10-14 in a single detector, but only Sh = 10-44 Hz-1 if two detectors were cross-correlated for one year. (Compare to LISA design Sh = 10-40 Hz-1 at 10 mHz.)

10. Foreground sources What sources might live in this band (cf Ungarelli & Vecchio)? • NS-NS coalescences, NS-BH/BH-BH coalescences for BH masses below 105 M. • Bursts from formation by collapse of 300-1000 M black holes (Fryer et al 2001). • Slow pulsars, magnetars. • Exotica, eg cosmic string kinks and cusps (Damour & Vilenkin 2001).

11. Chirps in the intermediate band

12. Chirp sensitivity of LISA follow-on instrument in intermediate band Assume Sh = 10-44 Hz-1 between 0.1 and 10 Hz, observation lasts up to one year, chirping binary at z = 1. log(SNR) Binary chirp mass (solar)

13. log(SNR) Binary chirp mass (solar) Chirp sensitivity of LISA follow-on instrument in intermediate band. II