Gamma ray burst triggered searches in the S2, S3, S4 runs

of LIGOSoumya D. Mohanty

On behalf of the LIGO Science Collaboration

Relevant astrophysics

• Central engine: GRBs come in two classes– Long soft (LSB): Stellar core collapse to Black Holes

• or magnetars for anomalously long and soft bursts, e.g., 060218, 1998bw (980425)

– Short hard (SHB): NS-NS, NS-BH, BH-WD mergers following GW driven inspiral

• Distance scales: – LSB should follow massive Star formation rate pdf

of observed redshifts peaks at z > 1– SHB pdf should peak at closer redshifts ( 0.5)

Detection Rate/upper limits

• Analysis pipelines developed for both direct coincidence and population signature searches

• Direct detection– LSB: about 1/yr for adv. LIGO (kobayashi, Meszaros, 2003)– SHB: ? (papers by Kalogera et al)– Very low redshift X-ray Flashes (D 100 Mpc): once in 3 to 5

years

• Upper limits on EGW

– 030329 (XRF) and S2: 340 M๏c2 ; sub-solar possible with initial LIGO (980425 was at 35 Mpc)

– GRB population average (Finn, Mohanty, Romano, 1999): sub-solar on LSBs possible with adv. LIGO

GRBs during S2, S3, S4 runs

• 39 GRBs (with 2 SHBs): – HETE-2, INTEGRAL, Swift triggers obtained

via GCN notices– 22 GRBs were well localized on the sky

• 35 GRBs had the two Hanford detectors in lock

• More …

Cross-correlation based search

• Cc statistic– Definition of cc– Max. over cc (max. over abs(cc) for LHO and

LLO)

• Parameters of the search– Integration length 100 ms– Search length 180 sec (-60 to +20 sec )

Significance

Binomial test

Upper limits

Search for population signature

• Max. Likelihood ratio detection statistic– Prior information about

redshifts, directions and astrophysics not taken into account yet

– Sum-max statistic

• Non-parametric version (rank-sum test) significance: 0.64 – Cannot reject null

hypothesis (no GWs associated with the S2, S3, S4 GRB sample)

Constraints on population properties

• Joint pdf of population parameters determines the distribution of the sum-max statistic– Population parameters:

redshift, EGW, inclination (beaming)

– Frequentist confidence belt

• S2,S3,S4: only H1,H2 cross-correlation used– Single source parameter:

observed matched filtering snr

TO BE REPLACED BY RECENTLY CHANGED CF BELTS

Population model

• Standard candle model

• Redshift distribution prediction from Bromm, Loeb, 2002– Zpeak = 1.8 (for Swift)

• 90% confidence level upper limit on EGW is 2.4104 M๏c2

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LIGO/GEO S5 run

• Isabel to fill in

Summary

• Search for GWs associated with GRBs conducted over S2, S3, S4 data; upper limits obtained (individual/population)

• Analysis pipelines in place• Better understanding of how to combine multiple

triggers: exploring parametric/non-parametric approaches

• S5 results for population upper limits may be 10 to 20 times better

• Many possibilities for enhancing sensitivity: shorter search lengths, coherent network analysis,…