Searching for accreting neutron stars C Messenger, V Re and A - - PowerPoint PPT Presentation

Searching for accreting neutron stars

C Messenger, V Re and A Vecchio

• n behalf of LSC-PULG

8th Gravitational Waves Data Analysis Workshop UWM, 17th – 20th December 2003

Outline

• Astrophysical scenario
• Data analysis

– General – Approach for LIGO S2 data set

• Work in progress and future plans

GWDAW8, 17th – 20th December 2003 A Vecchio – Accreting neutron stars

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)

GWDAW8, 17th – 20th December 2003 A Vecchio – Accreting neutron stars

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:

1. Density fluctuations – “mountain”

• n neutron star (Bildsten, 1998;

Ushomirsky, Cutler, Bildsten, 2000; Cutler, 2002)

– fgw = 2 frot

2. R-modes (Andersson et al, 1999; Wagoner, 2002)

– fgw = 4/3 frot

(from Cutler and Thorne 2000)

GWDAW8, 17th – 20th December 2003 A Vecchio – Accreting neutron stars

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 < 150 sec (Sco X-1)
• .∆f ~ 0.2 Hz (Sco X-1)

P: binary period q = m2/mNS

GWDAW8, 17th – 20th December 2003 A Vecchio – Accreting neutron stars

Data analysis (cont’d)

• 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 < 2 weeks

• Rotation frequency, and therefore GW frequency, not very

well known: ∆f ~ 1 – 40 Hz

τ: time scale

• ver which

torque doubles

• r turns off

GWDAW8, 17th – 20th December 2003 A Vecchio – Accreting neutron stars

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: simplest approach is to use a “stack-slide” search (Brady and T Creighton, 1999)

• Coherent integration over Tc
• Concatenate incoherently M chunks of length Tc (total
• bservation time = M Tc)

– Stack-slide search code development is well underway

GWDAW8, 17th – 20th December 2003 A Vecchio – Accreting neutron stars

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

• Only coherent analysis (the core of the entire

search strategy) over:

– Relevant band for emission at twice the rotation frequency (but if enough processing power is available we’ll explore band for 4/3 rotation frequency) – The longest possible observation time for available computational resources (Tsunami: 200 CPUs)

T(coherent) < 1/2 day

• Frequency domain analysis (different flavour of

analysis carried out for S1)

GWDAW8, 17th – 20th December 2003 A Vecchio – Accreting neutron stars

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: ∆α = 0.1 rad
• 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

GWDAW8, 17th – 20th December 2003 A Vecchio – Accreting neutron stars

S2 analysis in progress

• One-stage coherent FD analysis (F statistic)
• Tobs = 6 hrs

– Same data segment for L1, H1 and H2

• Search carried out over:

– Wide frequency band

1. “lower band”: 464 – 484 Hz 2. “upper band”: 604 – 624 Hz

– Discrete mesh over 2D parameter space (10% mismatch)

• Period known to high accuracy (not a search parameter for

Tobs < 1 month)

• Mesh on a and α: 39,487 filters (upper-band)
• Frequentist upper-limits on the relevant frequency

range

GWDAW8, 17th – 20th December 2003 A Vecchio – Accreting neutron stars

Tsunami 200 CPUs

Limit on integration time

• 3 detectors
• Sco X-1

parameters

• 40 Hz

bandwidth

GWDAW8, 17th – 20th December 2003 A Vecchio – Accreting neutron stars

Conclusions

• Coherent search analysis pipeline is in place
• S2 analysis is in progress
• However, sensitivity improvements rely on

hierarchical search approach (well underway – dedicated effort for “stack-slide” in PULG by Landry & Mendell)

• Future plans

– Full stack-slide analysis code ready for S3 analysis – Place upper-limits on other accreting neutron stars – Place upper-limits on emission at 4/3 frot

GWDAW8, 17th – 20th December 2003 A Vecchio – Accreting neutron stars