High frequency gw sources
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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 [email protected] Ground-based GW Astronomy.

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High-Frequency GW Sources

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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

and

Cardiff University, Cardiff, UK

http://www.aei.mpg.de

[email protected]


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

Crab

log(h)

J1540-6919

J1952+3252

LIGO I

J0437-4715

LIGO II

J1744-1134

log(f/Hz)

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).


Chirps in the intermediate band

Chirps in the intermediate band


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.

log(SNR)

Binary chirp mass (solar)


Chirp sensitivity of lisa follow on instrument in intermediate band ii

log(SNR)

Binary chirp mass (solar)

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


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