searching for gravitational radiation from scorpius x-1: limits from the second ligo science run
DESCRIPTION
Searching for gravitational radiation from Scorpius X-1: Limits from the second LIGO science run. Alberto Vecchio on behalf of the LIGO Scientific Collaboration GWDAW10 - UTB, 14th - 17th December, 2005 LIGO - G05xxxx. Outline. Analyses carried out by the LSC Pulsar group - PowerPoint PPT PresentationTRANSCRIPT
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Searching for gravitational radiation from Scorpius X-1: Limits from the second LIGO
science run
Alberto Vecchio
on behalf of the LIGO Scientific Collaboration
GWDAW10 - UTB, 14th - 17th December, 2005
LIGO - G05xxxx
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Outline
• Analyses carried out by the LSC Pulsar group• Coherent wide parameter space searches:
several astrophysical targets– This talk: Scorpius X-1 (low-mass X-ray binary)
• Second LIGO science run (S2)• Method and results from the Sco X-1 analysis
using S2 data• Ongoing work and future plans
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LSC pulsar searches• Search strategies (and pipelines) for continuous GWs:
– Time domain analysis: optimal for “single filter” (or very limited parameter space) targets, such as radio pulsars
• e.g. S2 analysis of 28 radio pulsars: PRL 94, 181103 (2005)
– Coherent matched-filter based frequency domain analysis: optimal sensitivity for large parameter space (but computationally intensive) - this talk
• “Same” software as that used in E@h (Bruce Allen’s talk)
– Semi-coherent analysis: computationally efficient, but reduced sensitivity • Hough transform: e.g. all-sky, fdot, isolated neutron stars analysis using S2 data:
PRD 72, 102004 (2005)• Power flux• Stack-slide
• We have been integrating these fundamental building blocks into hierarchical pipelines: best sensitivity for fixed computational resources
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Coherent analyses• Previous analyses:
– S1 analysis targeted at PSR J1939+2134 (“single filter”): h095% = 1.4 0.1 x
10-22 (PRD 69, 082004 (2004))– Explorer data: all-sky, fdot, 0.76 Hz frequency band 922 Hz: h0
90% = 1.0 x 10-
22 (Astone et al, 2005)
• Analyses of (a small sub-set of) S2 LIGO data:– Wide parameter space
• Blind search for isolated neutron stars (all sky, no spindown + frequency band 160 -728.8 Hz) [presented at Amaldi6]
• Scorpius X-1: low-mass X-ray binary (orbital parameters + frequency bands 464 - 484 HZ & 604 - 624 Hz)
– Search “in coincidence” (H1 and L1 data)– Lots of technical and implementation issues
• Notice: the “time domain pipeline” is a coherent pipeline designed in a different framework and (currently) not suitable to probe a wide parameter space
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Accreting neutron stars
Braking mechanism is needed to explain lack of sourceshere
Braking due to GW’s (Bildsten, 1998):I. Accretion replenishes angular momentum lost to GWsII. GW torque scales as 5: fairly insensitive to physical parameters
clustering of frequenciesIII. Brightest X-ray source is also brightest GW source: Sco X-1
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S2 LIGO Science Run
• S2: Feb. - Apr. 2003: 59 days • Duty cycle:
–H1: 74%–H2: 58%–L1: 37%
Predicted Sco X-1 signal amplitude:
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Scorpius X-1
Known parameters:
Sky position (no search over sky)
Period (for T < 1 month)
No spin-down (for T < 1 week)
Search parameters:
Orbit: projected semi-major axis and “time of periapse passage”
Frequency
Possibly eccentricity (we set e = 0 in templates for S2 search)
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Computational costs
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For Tspan < orbital period, the time of the analysis scales as:
This is a computationally limited search
€
ΔfΔaΔT( ) f02Tspan
7
103 104 105 106
Observation time Tspan /sec
1018
1014
1012
1010
108
106
104
102
Number of templates S2 analysis
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Sco X-1 S2 analysis• Coherent analysis: F-statistic (Jaranowski, Krolak,
and Schutz, 1998)• Integration time: the “best” 6 hours of data• L1 and H1 in coincidence• Parameter space (3-dimensional):
– 1- region of the two relevant orbital parameters– 40 Hz frequency band (464 - 484 Hz and 604 - 624 Hz)
• Frequentist upper-limit based on joint significance
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Signal model
GW phase (in source frame): monochromatic over the observation time
Time transformations:
From SSB time T to detector time t (depends on position in the sky):
From SSB time T to source time tp (depends on orbital parameters)
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Pipeline
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Data Selection
• SFTs: 60 sec• Two 20 Hz bands
• Use best data segment with Tspan < 6 h Source positionNoise floorAmount of data with Tspan
464 - 484 Hz
604 - 624 Hz
L1
H1
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Filter bank
• Number of filters (2 orbital param. x frequency) for detectors and bands:– L1: 464 - 484 Hz : (3 391 - 3 668) x 1 875 500– L1: 604 - 624 Hz : (5 738 - 6107) x 1 875 500 – H1: 464 - 484 Hz : (6 681 - 7 236) x 2 154 000– H1: 604 - 624 Hz : (11 309 - 12 032) x 2 154 000
• 2 dimensional filter banks (orbit) + frequency
• Overall max mismatch 0.1 • Orbital bank (using metric) for
each detector and 1 Hz wide band
• Frequency sampled at 1/(5 Tspan)
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1/50 of the whole parameter space
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Single IFO results
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465 Hz - 466 HzExample of expected vs actual distribution of the values of 2F in single interferometer output
“Good” band
“Bad” band (not the worse!)
L1
H1
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Coincidence analysis
Illustration of coincidence analysis (orbital filters)
We test for templates in H1 and L1 to be consistent with same astrophysical signal:
a. Orbital parameters
b. Frequency: 40 frequency bins
Orbital parameter 1
Orb
ital p
ara
me
ter
2
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Coincidence analysisBefore coincidence
After coincidence
464 - 484 Hz 604 - 624 HzMean value of loudest 2F (excluding 3 disturbed bands) for each 1 Hz band before and after coincidence:
<2 F>L1 = 40.8
<2 F>L1 = 28.6
<2 F>H1 = 45.4
<2 F>H1 = 33.5
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Coincident templates:(T,a) plane
614-615 Hz
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Coincident templates:(a,f) plane
614-615 Hz
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Setting an upper limit
• Coincident templates at the end of the pipeline are ranked according to their joint significance:
• We inject in software a population of signals with constant amplitude h0 and random parameters (orbit, frequency and nuisance parameters) and detect them with the same pipeline
• The upper-limit (on amplitude) h095% corresponds to the amplitude of
the population that in 95% of the injections produces a more significant coincidence than the most significant coincident template found in the actual analysis
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Preliminary upper limits (e = 0)
464 - 484 Hz 604 - 624 Hz
h095% = 2 x 10-22
Errors:
• MonteCarlo: 1-3%
• Calibration: 9%
Best upper-limit:
h095% = 1.73 (0.02 + 0.16) x 10-22
Upper limit on ellipticity (assuming Sco X-1 distance of 2.8 kpc):
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469-470
464-466
614-615
479-480
e = 5x10-3
10-3
5x10-4
0 10-4
h0 vs confidence (preliminary)
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Preliminary upper limits (e 0)
e = 10-4 e = 5 x 10-4 e = 10-3 e = 5 x 10-3
C = 0.95 C = 0.95 C = 0.88 C = 0.50
For e > 5 x 10-4 the pipeline looses efficiency: the fitting factor of templates with signal is < 1
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Conclusions• Coherent analyses in coincidence of S2 data over a wide parameter
space:– Blind search for isolated neutron stars over 10 hours of data: all sky, no
spindowns, frequency band 160 -728.8 Hz– Scorpius X-1 over 6 hours of data: orbital parameters and frequency bands 464
- 484 Hz & 604 - 624 Hz
• First (preliminary) upper limit on gravitational radiation from Sco X-1: h0
95% ~ 2 x 10-22 and 95% ~ 5 x 10-4
• Future work:– Pipeline: incorporated as coherent stage into hierarchical search schemes– Accreting neutron stars:
• Same pipeline used to target X-ray accreting milli-second neutron stars (source parameters much better known) over whole S4/5
• Hierarchical search using whole S4/5 aimed at Sco X-1 and the other low mass x-ray binaries
• How close can we get to the predicted signal strength?