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

LIGO-G030585-00-Z
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Outline

• Astrophysical scenario
• Data analysis
• General
• S2 approach
• S2 preliminary results
• Future work

LSC Meeting, LHO 10th 13th November, 2003

LIGO-G030585-00-Z
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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 are the proposed mechanisms
• Magnetic braking (Wang and Zang, 1997), but need
  for fine tuning of parameters
• Gravitational waves (Bildsten, 1998)

LSC Meeting, LHO 10th 13th November, 2003

LIGO-G030585-00-Z
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GWs from accreting neutron stars

• Conjecture for LMXBs GWs are the limiting
  physics that prevents NSs from being spun-up to
  the braking frequency
• Two models
• Density fluctuations 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

(from Cutler and Thorne 2000)
LSC Meeting, LHO 10th 13th November, 2003

LIGO-G030585-00-Z
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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
• Signal confined to a single bin for Tobs lt 150
  sec (Sco X-1)
• .Df 0.2 Hz (Sco X-1)

P binary period q m2/mNS
LSC Meeting, LHO 10th 13th November, 2003

LIGO-G030585-00-Z
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Data analysis (cont)

• Intrinsic change in GW frequency (spin down)
• System is in equilibrium GW emission balances
  accretion torque frequency makes a random walk
  as the accretion rate (Mdot) changes in time
• We can not model this frequency evolution using
  low-order polinomial in time
• However, the signal is confined to a single
  frequency by for Tobs lt 2 weeks
• Rotation frequency, and therefore GW frequency,
  not very well known Df 1 40 Hz

t time scale over which torque doubles or turns
off
LSC Meeting, LHO 10th 13th November, 2003

LIGO-G030585-00-Z
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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 106 for 1
  day of coherent integration)
• Long integration times
• Search strategy
• Hierarchical (Hough transform/stack-slide)
• Coherent integration over Tc
• Concatenate incoherently M chunks of length Tc
  (total observation time M Tc)
• Possibly run on the Grid

LSC Meeting, LHO 10th 13th November, 2003

LIGO-G030585-00-Z
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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
• Relevant band for emission at twice the rotation
  frequency (but if enough processing power is
  available well explore band for 4/3 rotation
  frequency)
• The longest possible observation time for
  available computational resources (Birmingham
  cluster Tsunami 200 CPUs)
• T(coherent) lt 1/2 day
• Frequency domain analysis (different flavour of
  analysis carried out for S1)

LSC Meeting, LHO 10th 13th November, 2003

LIGO-G030585-00-Z
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Schematic of pipeline
Sco X-1 coordinates
Data 60 sec long FFTs
Demodulation Compute F statistic
Sco X-1 orbital parameters
Template bank
Vetoes Select candidates
Upper-limit
S2 preliminary results
Monte Carlos
LSC Meeting, LHO 10th 13th November, 2003

LIGO-G030585-00-Z
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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 lt a lt 2.213
  sec
• Initial orbital phase Da 0.1 rad
• Circular orbit (e lt 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

LIGO-G030585-00-Z
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S2 preliminary analysis

• One-stage coherent FD analysis (F statistic)
• Tobs 3 hrs (set by Tanalysis 1 week on 100
  CPUs)
• Same data segment for L1, H1 and H2
• Search carried out over
• Wide frequency band
• lower band 464 484 Hz
• upper band 604 624 Hz
• Discrete mesh over 2D parameter space (10
  mismatch)
• Period known to high accuracy (not a search
  parameter for Tobs lt 1 month)
• Mesh on a and a 39,487 filters (upper-band)
• Frequentist upper-limits on ten 4 Hz wide bands
  covering the relevant frequency range

LSC Meeting, LHO 10th 13th November, 2003

LIGO-G030585-00-Z
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Choice of data segment
High sensitivity segment during S2 for all the
three detectors in both bands GPS times
734238015 lt t lt 734249072 corresponding to Tobs
11,057 sec
464 484 Hz
604 624 Hz
H1
H2
L1
S2
S2
LSC Meeting, LHO 10th 13th November, 2003

LIGO-G030585-00-Z
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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

LIGO-G030585-00-Z
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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

LIGO-G030585-00-Z
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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)

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

LIGO-G030585-00-Z
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Summary

• Angle-averaged upper-limit at 95 confidence is
  h0(95) 5x10-22 - 3x10-21 depending on
  detector and frequency band (but only 2 frot)
• 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 are implementing the test currently being used
  for isolated pulsars

LSC Meeting, LHO 10th 13th November, 2003

LIGO-G030585-00-Z
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Work plan

• November
• Include vetoing stage
• Re-run analysis
• Perform additional tests/checks on the entire
  pipeline
• Make final
• Trade-offs
• Search optimisation
• Select Tobs for S2 and data segment
• December
• Run S2 analysis

LSC Meeting, LHO 10th 13th November, 2003

LIGO-G030585-00-Z