Talk overview Introduction to continuous wave (CW) sources

НазваTalk overview Introduction to continuous wave (CW) sources
Дата конвертації06.06.2013
Розмір691 b.

The search for continuous gravitational waves: analyses from LIGO’s second science run Michael Landry LIGO Hanford Observatory on behalf of the LIGO Scientific Collaboration April APS Meeting (APR04) May 1-4, 2004 Denver, CO

Talk overview

  • Introduction to continuous wave (CW) sources

  • CW search group analysis efforts

  • Review of first science run (S1) results, and a look at expectations of the S2 run

  • Time-domain analysis method

  • Injection of fake pulsars

  • Results

CW sources

  • Nearly-monochromatic continuous sources of gravitational waves include neutron stars with:

    • spin precession at ~frot
    • excited oscillatory modes such as the r-mode at 4/3 * frot
    • non-axisymmetric distortion of crystalline structure, at 2frot
  • Limit our search to gravitational waves from a triaxial neutron star emitted at twice its rotational frequency (for the analysis presented here, only)

  • Signal would be frequency modulated by relative motion of detector and source, plus amplitude modulated by the motion of the antenna pattern of the detector

Source model

  • F+ and Fx : strain antenna patterns of the detector to plus and cross polarization, bounded between -1 and 1

  • Here, signal parameters are:

    • h0 – amplitude of the gravitational wave signal
    •  – polarization angle of signal
    •  – inclination angle of source with respect to line of sight
    • 0 – initial phase of pulsar; (t=0), and (t)= t0

CW search group efforts

  • S2 Coherent searches

    • Time-domain method (optimal for parameter estimation)
      • Target known pulsars with frequencies (2frot) in detector band
    • Frequency-domain F-statistic* method (optimal for blind detection)
      • All-sky, broadband search, subset of S2 dataset
      • Targeted searches (e.g. galactic core)
      • LMXB (e.g. ScoX-1) search
  • S2 Incoherent searches

    • Hough transform method
    • Powerflux method
    • Stackslide method
  • Future: Implement hierarchical analysis that layers coherent and incoherent methods

  • Einstein@home initiative for 2005 World Year of Physics

First science run: S1

  • S1 run: 17 days (Aug 23-Sep 9 02)

  • Coincident run of four detectors, LIGO (L1, H1, H2), and GEO600

  • Two independent analysis methods (frequency-domain and time-domain) employed

  • Set 95% upper limit values on continuous gravitational waves from single pulsar PSR J1939+2134, using LIGO and GEO IFO’s: best limit from Livingston IFO:

S2 expectations

  • Coloured spectra: average amplitude detectable in time T (1% false alarm, 10% false dismissal rates):

Time-domain analysis method

  • Perform time-domain complex heterodyne (demodulation) of the interferometer gravitational wave channel

  • Low-pass filter these data

  • The data is downsampled via averaging, yielding one value (“Bk”) of the complex time series, every 60 seconds

  • Determine the posterior probability distribution (pdf) of the parameters, given these data (Bk) and the model (yk)

  • Marginalize over nuisance parameters (cos0) to leave the posterior distribution for the probability of h0 given the data, Bk

  • We define the 95% upper limit by

  • a value h95 satisfying:

Bayesian analysis

  • A Bayesian approach is used to determine the posterior

  • distribution of the probability of the unknown parameters via the

  • Likelihood (assuming gaussian noise within our narrow band):

Marginalizing over noise

  • As we estimate the noise level from the Bk no independent information is lost by treating it as another nuisance parameter over which to marginalize, i.e.

Analysis summary

S2 hardware signal injections

  • Performed end-to-end validation of analysis pipeline by injecting simultaneous fake continuous-wave signals into interferometers

  • Two simulated pulsars were injected in the LIGO interferometers for a period of ~ 12 hours during S2

  • Fake signal is sum of two pulsars, P1 and P2

  • All the parameters of the injected signals were successfully inferred from the data

Preliminary results for P1

  • Parameters of P1:

Preliminary results for P2

  • Parameters for P2:

Pulsar timing

  • Analyzed 28 known isolated pulsars with 2frot > 50 Hz.

    • Timing information has been provided using radio observations collected over S2/S3 for 18 of the pulsars (Michael Kramer, Jodrell Bank).
    • Timing information from the Australia Telescope National Facility (ATNF) catalogue used for 10 pulsars
  • An additional 10 isolated pulsars are known with 2frot > 50 Hz but the uncertainty in their spin parameters is such that a search over frequency is warranted

  • Crab pulsar heterodyned to take timing noise into account

Preliminary results for PSR B0021-72L

Preliminary results for the Crab pulsar

Preliminary upper limits for 28 known pulsars

Equatorial Ellipticity

  • Results on h0 can be interpreted as upper limit on equatorial ellipticity

  • Ellipticity scales with the difference in radii along x and y axes

Preliminary ellipticitylimits for 28 known pulsars

Summary and future outlook

  • S2 analyses

    • Time-domain analysis of 28 known pulsars complete
    • Broadband frequency-domain all-sky search underway
    • ScoX-1 LMXB frequency-domain search near completion
    • Incoherent searches reaching maturity, preliminary S2 results produced
  • S3 run

    • Time-domain analysis on more pulsars, including binaries
    • Improved sensitivity LIGO/GEO run
    • Oct 31 03 – Jan 9 04
    • Approaching spindown limit for Crab pulsar


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