Gamma ray burst triggered searches in the S2, S3, S4 runs of LIGO Soumya D. Mohanty On behalf of the...

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

2/3

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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,…