Upper-limit on Sco X-1 S2 preliminary results

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

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

3. Astrophysical scenario • 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) (from Cutler and Thorne 2000) LSC Meeting, LHO 10th – 13th November, 2003

4. 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: • “Mountain” on neutron star (Bildsten, 1998; Ushomirsky, Cutler, Bildsten, 2000; Cutler, 2002) • fgw = 2 frot • R-modes (Andersson et al, 1999; Wagoner, 2002) • fgw = 4/3 frot LSC Meeting, LHO 10th – 13th November, 2003

5. 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 Tobs > 5 min (Sco X-1) • .Df ~ 0.2 Hz (Sco X-1) LSC Meeting, LHO 10th – 13th November, 2003

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

7. 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 ~ 106 for 1 day of coherent integration) • In the long term the only viable search strategy is: • Hierarchical • On the Grid (Dhurandhar and Vecchio, 2001) LSC Meeting, LHO 10th – 13th November, 2003

8. 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 10th – 13th November, 2003

9. 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 < a < 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 10th – 13th November, 2003

10. 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 Tobs < 1 month) • Mesh on a and a • Tobs ~ 3 hrs (set by Tanalysis = 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 10th – 13th November, 2003

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

12. Choice of data segment • Highest sensitivity segment during S2 for all the three detectors in both bands • GPS times: 734249072 < t < 734238015 • Tobs = 11,057 sec 464 – 484 Hz 604 – 624 Hz H1 H2 L1 LSC Meeting, LHO 10th – 13th November, 2003

13. h0 = 10-21 h0 = 10-21 h0 = 10-21 Lower band : 464 – 476 Hz • 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 LSC Meeting, LHO 10th – 13th November, 2003

14. h0 = 10-21 h0 = 10-21 Lower band: 476 – 484 Hz • Extremely noisy band • Strongly contaminated by broad feature at 480 Hz • 2F* ~ 100 – 1200 depending on IFO LSC Meeting, LHO 10th – 13th November, 2003

15. h0 = 10-21 h0 = 10-21 h0 = 10-21 h0 = 10-21 h0 = 10-21 Upper band: 604 – 624 Hz • Quiet band • Results consistent with noise only (2F* ~ 50) for L1 and H1 • H2 is slightly noisier (2F* = 55-60)

16. Summary of preliminary UL h0 = 3x10-21 h0 = 2x10-21 h0 = 10-21 h0 = 5x10-22 LSC Meeting, LHO 10th – 13th November, 2003

17. Summary • h0(95%) = 5x10-22 - 3x10-21 for the three detectors over the entire bandwidth (2 frot) • No filter triggers “in coincidence” • No analysis has been carried out on the band corresponding to 4/3 frot • 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 10th – 13th November, 2003

18. 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 10th – 13th November, 2003