High-Frequency GW Sources Bernard F Schutz Albert Einstein Institute – Max Planck Institute for Gravitational Physics, Golm, Germany and Cardiff University, Cardiff, UK

Bernard F Schutz Albert Einstein Institute 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 ~ , 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/ (GR16 Proceedings). Focus in this talk on two topics: spinning neutron stars, and sources in the intermediate frequency band ( Hz).

Bernard F Schutz Albert Einstein Institute 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!

Bernard F Schutz Albert Einstein Institute 4 log(f/Hz) log(h) LIGO I LIGO II 1  noise in 1-year observation Upper limits on some known pulsars J Crab J J J

Bernard F Schutz Albert Einstein Institute 5 How could pulsars radiate? Crustal asymmetries. Cutler & Thorne: LIGO II will see any with ellipticity e > 2 x (f kHz ) -2 r kpc. 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 < 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, …)

Bernard F Schutz Albert Einstein Institute 6 New mechanism: toriodal B-field flip Pulsar B-fields not understood, but dynamos require toroidal fields B t. 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. Cutler (gr-qc/ ) adds new twist: B t 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 B t 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.

Bernard F Schutz Albert Einstein Institute 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 B p is not perpendicular to B t (cf Earth field offset angle).

Bernard F Schutz Albert Einstein Institute 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.

Bernard F Schutz Albert Einstein Institute 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 S h = Hz -1 to reach  gw = in a single detector, but only S h = Hz -1 if two detectors were cross-correlated for one year. (Compare to LISA design S h = Hz -1 at 10 mHz.)

Bernard F Schutz Albert Einstein Institute 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 10 5 M . Bursts from formation by collapse of M  black holes (Fryer et al 2001). Slow pulsars, magnetars. Exotica, eg cosmic string kinks and cusps (Damour & Vilenkin 2001).

Bernard F Schutz Albert Einstein Institute 11 Chirps in the intermediate band

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

Bernard F Schutz Albert Einstein Institute 13 Chirp sensitivity of LISA follow-on instrument in intermediate band. II Binary chirp mass (solar) log(SNR)