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Upper-limit on Sco X-1 S2 preliminary results C Messenger, V Re and A Vecchio on behalf of PULG LSC General Meeting LHO, 10 th – 13 th November 2003

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Outline Astrophysical scenario Data analysis –General –S2 approach S2 preliminary results Future work –Open issues –Plan for final upper-limits LSC Meeting, LHO 10 th – 13 th November, 2003

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Astrophysical scenario (from Cutler and Thorne 2000) Observational evidence that rotation frequencies in Low Mass X-Ray Binaries (LMXB) are –Well below NS breaking frequency –Clustered in a narrow (237 Hz – 619 Hz) frequency range (Bildsten, 1998; Chakrabarty et al, 2003) Currently, two models: –Magnetic braking (Wang and Zang, 1997), but problems with lack of persistent emission and need for fine tuning of parameters –Gravitational waves (Bildsten, 1998) LSC Meeting, LHO 10 th – 13 th November, 2003

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GWs from accreting neutron stars Accretion onto neutron stars considered since the Seventies a viable mechanism to sustain GW emission (Papaloizou and Pringle, 1978; Wagoner, 1984) Conjecture for LMXBs: GWs are the limiting physics that prevent NSs from being spun-up to the braking frequency –Two models: 1.“Mountain” on neutron star (Bildsten, 1998; Ushomirsky, Cutler, Bildsten, 2000; Cutler, 2002) –f gw = 2 f rot 2.R-modes (Andersson et al, 1999; Wagoner, 2002) –f gw = 4/3 f rot LSC Meeting, LHO 10 th – 13 th November, 2003

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Data analysis Source position: known Orbital motion –Circular orbit to a very good approximation –Search with discrete mesh over 3 orbital parameters (period, projected orbit semi-major axis, initial phase) –Phase Doppler shift much more severe than for isolated sources Need to start correcting for T obs > 5 min (Sco X-1). f ~ 0.2 Hz (Sco X-1) LSC Meeting, LHO 10 th – 13 th November, 2003

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Data analysis (con’t) Frequency drift (spin down/up: GWs balance accretion torque): signal monochromatic for T obs ~ 2 weeks Rotation frequency not very well known: f ~ 1 – 40 Hz LSC Meeting, LHO 10 th – 13 th November, 2003

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Computationally bound search Computational load comes from two poorly constrained set of parameters: –Emission frequency: search over a fairly large bandwidth (tens of Hz) –Orbital parameters (Sco X-1: N_filt ~ 10 6 for 1 day of coherent integration) In the long term the only viable search strategy is: –Hierarchical –On the Grid LSC Meeting, LHO 10 th – 13 th November, 2003 (Dhurandhar and Vecchio, 2001)

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Approach for S2 analysis Target Sco X-1 (the brightest source) –The analysis can be extended in a straightforward way (in principle) to the other LMXBs Coherent analysis over: –The full bandwidth (for emission at twice the rotation frequency) –The longest possible observation time for available computational resources (B’ham cluster Tsunami: 200 CPUs) T (coherent) < 1 day Frequency domain analysis (different flavour of analysis carried out for S1) LSC Meeting, LHO 10 th – 13 th November, 2003

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Scorpius X-1 Neutron star in a binary system accreting from a low-mass (0.42 M_sun) companion –Distance: 2.8 (+/- 0.2) kpc –Orbital parameters Period: 0.787313(1) day = 18.9 hrs Projected semi-major axis: 1.990 sec < a < 2.213 sec “Initial” orbital phase: 1.21 < < 1.31 Circular orbit (e < 10-3) –Rotation frequency from twin kHz QPOs (van der Klis et al, 1997; van der Klis, 2000) 232 – 242 Hz 302 – 312 Hz LSC Meeting, LHO 10 th – 13 th November, 2003

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Preliminary analysis Analysis: one-stage coherent demodulation (F statistic) Frequency band –“lower band”: 464 – 484 Hz –“upper band”: 604 – 624 Hz Two dimensional parameter space –Period known to high accuracy (not a search parameter for T obs < 1 month) –Mesh on a and T obs ~ 3 hrs (set by T analysis = 1 week on 100 CPUs) –Same data segment for L1, H1 and H2 Frequentist upper-limits on ten 4 Hz wide bands covering the entire frequency range LSC Meeting, LHO 10 th – 13 th November, 2003

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Filter mesh Flat 2D mesh (suitable re- parameterisation of the search parameters a and ) on X and Y Mis-match: 10% Number of filters –39,487 (for upper band) –Factor ~2 fewer for lower band LSC Meeting, LHO 10 th – 13 th November, 2003

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Choice of data segment LSC Meeting, LHO 10 th – 13 th November, 2003 Highest sensitivity segment during S2 for all the three detectors in both bands GPS times: 734249072 < t < 734238015 Tobs = 11,057 sec 464 – 484 Hz604 – 624 Hz H1 H2 L1

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Lower band : 464 – 476 Hz LSC Meeting, LHO 10 th – 13 th November, 2003 h 0 = 10 -21 464 – 468 Hz: fairly noisy band in all 3 IFOs (2F* ~ 66 – 116) 468 – 476 Hz is a quiet band: results consistent with noise only (2F* ~ 50) for L1, H1 and H2

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Lower band: 476 – 484 Hz h 0 = 10 -21 LSC Meeting, LHO 10 th – 13 th November, 2003 Extremely noisy band Strongly contaminated by broad feature at 480 Hz 2F* ~ 100 – 1200 depending on IFO

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Upper band: 604 – 624 Hz h 0 = 10 -21 Quiet band Results consistent with noise only (2F* ~ 50) for L1 and H1 H2 is slightly noisier (2F* = 55- 60)

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Summary of preliminary UL h 0 = 2x10 -21 h 0 = 3x10 -21 h 0 = 10 -21 h 0 = 5x10 -22 LSC Meeting, LHO 10 th – 13 th November, 2003

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Summary h 0 (95%) = 5x10 -22 - 3x10 -21 for the three detectors over the entire bandwidth (2 f rot ) –No filter triggers “in coincidence” No analysis has been carried out on the band corresponding to 4/3 f rot Main limitation of current analysis –Lack of veto procedure (crucial for f ~ 480 Hz) –We will implement and apply the test currently being used for isolated pulsars LSC Meeting, LHO 10 th – 13 th November, 2003

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Work plan November –Perform a variety of tests and checks on the entire pipeline –Run analysis on much smaller bandwidth and longer observation time (main purpose: investigate run times) –Investigate trade-offs and perform optimisation of the search December –Run S2 analysis LSC Meeting, LHO 10 th – 13 th November, 2003

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