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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. Ground-based GW Astronomy.

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high frequency gw sources

High-Frequency GW Sources

Bernard F Schutz

Albert Einstein Institute – Max Planck Institute for Gravitational Physics, Golm, Germany


Cardiff University, Cardiff, UK



ground based gw astronomy
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).
spinning neutron stars
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!
upper limits on some known pulsars

1s noise in

1-year observation










Upper limits on some known pulsars
how could pulsars radiate
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, …)
new mechanism toriodal b field flip
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.
natural pulsar model
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).
searching for pulsars
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.
intermediate band sources
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.)
foreground sources
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).
chirp sensitivity of lisa follow on instrument in intermediate band
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.


Binary chirp mass (solar)

chirp sensitivity of lisa follow on instrument in intermediate band ii


Binary chirp mass (solar)

Chirp sensitivity of LISA follow-on instrument in intermediate band. II