GravitationalWaves (GWs) are expected to be emitted in association with Gamma-Ray Bursts (GRBs). In this context, we are analyzing data collected by the Virgo interferometer during 2005 to develop a method aimed to search for coincidences between GW bursts and GRBs. Our analysis is currently focused on Virgo C7 run and the long GRB 050915a, observed by the Swift satellite. The goal of such analysis is either to identify significant events around the GRB trigger time or, in the absence of such events, to set a limit on the strength of the associated GW emission. This study is a prototype for evaluating Virgo capability in constraining the GW output associated with a typical long GRB. Here we give an overview of the procedure we are following in our analysis

Acernese, F.

Alshourbagy, M.

Amico, P.

Antonucci, F.

Aoudia, S.

Astone, P.

Avino, S.

Babusci, D.

Ballardin, G.

Barone, F.

Barsotti, L.

Barsuglia, M.

Bauer, Th.S.

Beauville, F.

Bigotta, S.

Birindelli, S.

Bizouard, M.A.

Boccara, C.

Bondu, François

Bosi, L.

Braccini, S.

Bradaschia, C.

Brillet, A.

Brisson, V.

Buskulic, D.

Calloni, E.

Campagna, E.

Carbognani, F.

Cavalier, F.

Cavalieri, R.

Cella, G.

Cesarini, E.

Chassande-Mottin, E.

Christensen, N.

Clapson, A.-C.

Cleva, F.

Corda, C.

Corsi, A.

Cottone, F.

Coulon, J.-P.

Cuoco, E.

Dari, A.

Dattilo, V.

Davier, M.

De Rosa, R.

Del Prete, M.

Di Fiore, L.

Di Virgilio, A.

Dujardin, B.

Eleuteri, A.

Evans, M.

Ferrante, I.

Fidecaro, F.

Fiori, I.

Flaminio, R.

Fournier, J.-D.

Frasca, S.

Frasconi, F.

Gammaitoni, L.

Garufi, F.

Genin, E.

Gennai, A.

Giazotto, A.

Giordano, G.

Giordano, L.

Gouaty, R.

Grosjean, D.

Guidi, G.

Hamdani, S.

Hebri, S.

Heitmann, H.

Hello, P.

Huet, D.

Karkar, S.

Kreckelbergh, S.

La Penna, P.

Laval, M.

Leroy, N.

Letendre, N.

Lopez, B.

Lorenzini, M.

Loriette, V.

Losurdo, G.

Mackowski, J.-M.

Majoranai, E.

Mantovani, M.

Marchesoni, F.

Marion, F.

Marque, J.

Martelli, F.

Masserot, A.

Mazzoni, M.

Menzinger, F.

Milano, L.

Moins, C.

Moreau, Julien

Morgado, N.

Mours, B.

N. Man, C.

Nocera, F.

van den Brand, F.J.

del Prete, M.

de Rosa, R.

Hal - Université Grenoble Alpes

GRB 050915a & the Virgo detector

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GRB 050915a & the Virgo detectorF. Acernese, P. Amico, M. Alshourbagy, F. Antonucci, S. Aoudia, P. Astone,S. Avino, D. Babusci, G. Ballardin, F. Barone, et al.To cite this version:F. Acernese, P. Amico, M. Alshourbagy, F. Antonucci, S. Aoudia, et al.. GRB 050915a & theVirgo detector. 42nd Rencontres de Moriond Gravitational Waves and Experimental Gravity,2007, La Thuile, Italy. <in2p3-00178917>HAL Id: in2p3-00178917http://hal.in2p3.fr/in2p3-00178917Submitted on 12 Oct 2007HAL is a multi-disciplinary open accessarchive for the deposit and dissemination of sci-entific research documents, whether they are pub-lished or not. The documents may come fromteaching and research institutions in France orabroad, or from public or private research centers.L’archive ouverte pluridisciplinaire HAL, estdestine´e au de´poˆt et a` la diffusion de documentsscientifiques de niveau recherche, publie´s ou non,e´manant des e´tablissements d’enseignement et derecherche franc¸ais ou e´trangers, des laboratoirespublics ou prive´s.GRB 050915a & the Virgo detectorF. ACERNESE6, P. AMICO10, M. ALSHOURBAGY11, F. ANTONUCCI 12, S. AOUDIA7, P.ASTONE12, S. AVINO6, D. BABUSCI4, G. BALLARDIN2, F. BARONE6, L. BARSOTTI11,M. BARSUGLIA8 , TH. S. BAUER13, F. BEAUVILLE1, S. BIGOTTA11, S. BIRINDELLI11,M.A. BIZOUARD8, C. BOCCARA9, F. BONDU7, L. BOSI10, C. BRADASCHIA11, S. BRACCINI11,J. F. J. VAN DEN BRAND13, A. BRILLET7, V. BRISSON8, D. BUSKULIC1, E. CALLONI6,E. CAMPAGNA3, F. CARBOGNANI2, F. CAVALIER8, R. CAVALIERI2, G. CELLA11,E. CESARINI3, E. CHASSANDE-MOTTIN7, N. CHRISTENSEN2, C. CORDA11, A. CORSI12,F. COTTONE10, A.-C. CLAPSON8, F. CLEVA7, J.-P. COULON7, E. CUOCO2, A. DARI10,V. DATTILO2, M. DAVIER8, M. DEL PRETE11, R. DE ROSA6, L. DI FIORE6, A. DI VIRGILIO11,B. DUJARDIN7, A. ELEUTERI6, M. EVANS2, I. FERRANTE11, F. FIDECARO11, I. FIORI2,R. FLAMINIO1,2, J.-D. FOURNIER7, S. FRASCA12, F. FRASCONI11, L. GAMMAITONI10,F. GARUFI6, E. GENIN2, A. GENNAI11, A. GIAZZOTTO11, G. GIORDANO4, L. GIORDANO6,R. GOUATY1, D. GROSJEAN1, G. GUIDI3, S. HAMDANI2, S. HEBRI2, H. HEITMANN7,P. HELLO8, D. HUET2, S. KARKAR1, S. KRECKELBERGH8, P. LA PENNA2, M. LAVAL7,N. LEROY8, N. LETENDRE1, B. LOPEZ2, LORENZINI3, V. LORIETTE9, G. LOSURDO3,J.-M. MACKOWSKI5, E. MAJORANA12, C. N. MAN7, M. MANTOVANI11, F. MARCHESONI10,F. MARION1, J. MARQUE2, F. MARTELLI3, A. MASSEROT1, M. MAZZONI3, L. MILANO6, F.MENZINGER2, C. MOINS2, J. MOREAU9, N. MORGADO5, B. MOURS1, F. NOCERA2,C. PALOMBA12, F. PAOLETTI2, 11, S. PARDI6, A. PASQUALETTI2, R. PASSAQUIETI11,D. PASSUELLO11, F. PIERGIOVANNI3, L. PINARD5, R. POGGIANI11, M. PUNTURO10,P. PUPPO12, S. VAN DER PUTTEN13, K. QIPIANI6, P. RAPAGNANI12, V. REITA9,A. REMILLIEUX5, F. RICCI12, I. RICCIARDI6, P. RUGGI 2, G. RUSSO6, S. SOLIMENO6,A. SPALLICCI7, M. TARALLO11, M. TONELLI11, A. TONCELLI11, E. TOURNEFIER1,F. TRAVASSO10, C. TREMOLA11, G. VAJENTE 11, D. VERKINDT1, F. VETRANO3, A. VICERE´3,J.-Y. VINET7, H. VOCCA10 and M. YVERT11Laboratoire d’Annecy-le-Vieux de physique des particules (LAPP), IN2P3/CNRS, Universite´ deSavoie, BP 110, F-74941, Annecy-le-Vieux, CEDEX, France;2European Gravitational Observatory (EGO), Via E. Amaldi, I-56021 Cascina (PI) Italia;3INFN - Sezione Firenze/Urbino Via G.Sansone 1, I-50019 Sesto Fiorentino; and/or Universita` diFirenze, Largo E.Fermi 2, I - 50125 Firenze and/or Universita` di Urbino, Via S.Chiara, 27 I-61029Urbino, Italia;4INFN, Laboratori Nazionali di Frascati Via E. Fermi, 40, I-00044 Frascati (Roma) - Italia;5LMA 22, Boulevard Niels Bohr 69622 - Villeurbanne- Lyon Cedex France;6INFN - Sezione di Napoli and/or Universita` di Napoli ”Federico II” Complesso Universitario di MonteS. Angelo Via Cintia, I-80126 Napoli, Italia and/or Universita` di Salerno Via Ponte Don Melillo,I-84084 Fisciano (Salerno), Italia;7Department Artemis - Observatoire de la Coˆte d’Azur, BP 42209, 06304 Nice Cedex 4, France;8Laboratoire de l’Acce´le´rateur Line´aire LAL, Univ Paris-Sud, IN2P3/CNRS, Orsay, France;9ESPCI - 10, rue Vauquelin, 75005 Paris - France;10INFN Sezione di Perugia and/or Universita` di Perugia, Via A. Pascoli, I-06123 Perugia - Italia;11INFN - Sezione di Pisa and/or Universita` di Pisa, Via Filippo Buonarroti, 2 I-56127 PISA - Italia;12INFN, Sezione di Roma and/or Universita` ”La Sapienza”, P.le A. Moro 2, I-00185, Roma - Italia;13National Institute for Nuclear Physics and High Energy Physics, NL-1009 DB Amsterdam and/orVrije Universiteit, NL-1081 HV Amsterdam, The NetherlandsGravitational Waves (GWs) are expected to be emitted in association with Gamma-Ray Bursts(GRBs). In this context, we are analyzing data collected by the Virgo interferometer during2005 to develop a method aimed to search for coincidences between GW bursts and GRBs.Our analysis is currently focused on Virgo C7 run and the long GRB 050915a, observed bythe Swift satellite. The goal of such analysis is either to identify significant events aroundthe GRB trigger time or, in the absence of such events, to set a limit on the strength ofthe associated GW emission. This study is a prototype for evaluating Virgo capability inconstraining the GW output associated with a typical long GRB. Here we give an overviewof the procedure we are following in our analysis.1 IntroductionGRBs are intense ﬂashes of γ-ray photons, lasting from some millisecond to several tens ofseconds. Depending on the duration of the prompt γ-ray emission, they have been classiﬁed intolong bursts, having typically durations greater than 2 s, and short ones, with durations below 2 s.GRBs are isotropically distributed in the sky, they originate at cosmological distances and theisotropic energy output in γ-rays can be as high as 1 Mc2. The sudden burst of γ-ray photonsis followed by a multi-wavelength (radio-to-X-rays) emission called “afterglow”, discovered bythe Italian-Dutch satellite BeppoSAX in 1997 1. The discovery of long GRB afterglows openedthe way to redshift measurements 2, that have deﬁnitely conﬁrmed the cosmological origin ofthese events. A similar major advance in GRB research was recently provided by the NASA’sSwift satellite, that, among other important results, has succeeded in localizing short GRBafterglows 3.Currently, the most favored scenario relates short and long GRBs to diﬀerent types of progen-itors (see e.g. 4 and references therein): long GRBs are thought to be produced by massive starsduring the explosions of their cores (so-called “collapsar” scenario), while short ones are believedto originate in compact binary mergers (such as Neutron Star (NS)-NS or Black Hole (BH)-NSmergers). In order to power a GRB, it is required that the progenitor stellar system evolvesending in the formation of a BH plus accretion disk, whose accretion onto the BH releases theenergy necessary to power an ultra-relativistic ﬂow, called “ﬁreball”. Dissipation of the ﬁre-ball kinetic energy, via shocks originating at distances larger than ∼ 1013 cm from the centralsource, explains the observed radiation. Due to the large distance, the central engine poweringthe GRB is in fact hidden from direct observation in the electromagnetic window. However, adirect signature of its identity may come from the gravitational window, since GWs should beproduced in the immediate neighborhood of the source, after the catastrophic stellar explosion.2 GBR 050915a and Virgo C7 runIn the context of the possible association between GRBs and GWs, we have started a work onGRB triggered searches for GW signals using the Virgo detector 5. A similar triggered searchwas performed by LIGO during its second science run, in coincidence with GRB 0303296. Virgois an Italian-French experiment 7. Jointly funded by INFN (Italy) and CNRS (France), it islocated at the European Gravitational Observatory (EGO), near Pisa. Virgo is an interferomet-ric device dedicated to detection of GWs. More speciﬁcally, it is a power recycled Michelsoninterferometer, with 3 km long arms. GWs result in a shift of the interference pattern producedby the interferometer, and thus in a change on the amount of light that is collected as its output.A recent review of Virgo status is given by 8.At the time of GRB 050915a, Virgo was engaged in a 5-day long data run, named C7. The“Burst Alert Telescope” (BAT) on-board Swift was triggered by GRB 050915a on the 15th ofSeptember 2005, at T= 11 : 22 : 42 UT. The BAT on-board calculated position for this burst wasRA=05h 26m 51s, Dec=−28d 01′ 48′′ (J2000), with an uncertainty of 3 arcmin. GRB 050915ahad a γ-ray (15− 350 keV) duration of (T90) 53± 3 s, making it a long-type GRB. The “X-rayTelescope” (XRT) began observing GRB 050915a at 11 : 24 : 09 UT (∼ 87 s after the trigger).The new reﬁned position was RA=05h 26m 44.6s, DEC=−28d 01m 01.0s 10.During C7 run, Virgo sensitivity exceeded that of any previous run. The lowest strain noisewas of ∼ 6 × 10−22 Hz−1/2, around ∼ 300 Hz. We have thus started performing an analysis ofVirgo data simultaneous with the long GRB 050915a, aimed to search for a burst-type GW signalassociated with this GRB. The redshift of this burst is not known, but considering that longGRBs in the Swift sample have typically z > 2 11, and taking into account current theoreticalestimates for the GW emission by the most likely GRB progenitors (e.g. 12), we expect thesearched signal in the Virgo detector to be very weak. In the absence of signiﬁcant events, theaim of our analysis is to set an upper-limit on the signal strength.3 Pipeline overviewFor our analysis, we relied on a single lock stretch (about 16845 s of data), containing the GRBtrigger time. A part from a time window 300 s long centered on the GRB trigger time, datawithin this lock stretch are used as “background”, i.e. they are used to determine the falsealarm rate and set a reasonable threshold for “good” events, to use when scanning the “signalregion”. The last is deﬁned as a data segment 180 s long, 120 s before the GRB trigger timeand 60 s after. Together with 60 s of data before its start and after its end, this data segmentmakes up the 300 s of the stretch which are not included in the background region. A 180 sduration for the signal region (roughly ten times the GRB duration) is chosen so to cover mostof astrophysical predictions regarding the expected delay between the GRB and the associatedburst-type GW signal.To search for burst-like events, we run the “Wavelet Detection Filter” (WDF, see 13,14)on both the background and signal region. The output of this ﬁlter is a list of events, eachcharacterized by a time, a duration and a signal-to-noise ratio (SNR, i.e. the event strength).For our analysis, only events having a SNR greater than 4 are considered. The ﬁlter has a timeresolution of 0.6 ms, that is the duration of a single temporal bin. If the SNR is above threshold(SNR=4) in two consecutive bins, and if the time delay between those two bins is less than agiven time interval called clusterization window (set to 10 ms in our analysis), then the binsare considered as part of the same event. In this way, we can associate to each event a timeduration, given by the time diﬀerence between the ﬁrst and last bin found to be part of thesame event. Among the bins recognized as part of the same event, the time of the one at whichthe SNR reaches its maximum, sets the event time.By running the WDF ﬁlter on the background region, we construct the distribution offalse alarm rate as a function of the event strength threshold and determine our good eventsthreshold, i.e. the SNR threshold needed to have an expected rate of false alarms of 5×10−4 Hz.A threshold of this kind, applied in a time window 180 s long (the duration of our signal region),corresponds to a ∼ 10% chance (assuming Poisson statistics) of ﬁnding a false coincidence withthe GRB trigger. When scanning the signal region in search for a GW burst coincident withthe GRB, we thus discard all events with SNR lower than this threshold, that in our case turnsout to be SNR=26.After selecting our good events threshold, we calibrated our pipeline for diﬀerent kind ofwaveforms by using software injections. Given the still large uncertainties in the predictions forGW signal waveforms expected to be associated with a long GRB, we have performed an analysisaimed to the detection of quite general types of GW bursts, like sine-Gaussians, Gaussians anddamped sinusoids. Typical durations of the injected signals are in the range 1 − 100 ms, withfrequencies between 203 Hz and 1503 Hz. The calibration procedure consists in two steps:ﬁrst, we characterize the ﬁlter eﬃciency in detecting signals at speciﬁc frequencies and withspeciﬁc durations, as a function of their strength; then, we deﬁne a conversion factor betweenthe detected event strength (SNR) and true event strength, that we quantify in terms of thesignal “root-sum-square” amplitude, hrss =∫+∞−∞ |h(t)|2 dt.Once concluded the calibration procedure on the background region, we scan the signalregion with the WDF ﬁlter and search for events above our good events threshold of SNR=26.We ﬁnd that in the source region there are no events with strength greater than this threshold,so a coincident search is prevented and we are going toward the deﬁnition of an upper-limitstrain amplitude. For each type of injected burst signal, our upper-limit strain amplitude willcorrespond to the hrss value giving a detected SNR of 264 ConclusionsWe have presented an overview of the analysis that is being performed on Virgo data takenduring the 2005 run named C7. Our work is aimed to search for a burst of GWs associatedwith the long GRB 050915a, that was detected by the Swift satellite when Virgo was takingdata in science mode. This is the ﬁrst time an analysis of this kind is being performed using theVirgo detector, thus its importance is crucial for establishing the level at which Virgo is actuallyable to constrain the GW output from GRB sources, but also as a prototype for future similaranalysis, that will be carried with improved sensitivity.References1. E. Costa et al., Nature, 387, 783 (1997).2. J. van Paradijs et al., Nature, 386, 686 (1997).3. N. Gehrels et al., Nature, 437, 851 (2005).4. P. Me´sza´ros, RPPh, 69, 2259 (2006).5. F. Acernese et al., CQG, submitted to, proceeding of GWDAW11 (2007).6. B. Abbott et al., Phys. Rev. D, 72, 042002 (2005).7. http://www.virgo.infn.it.8. F. Acernese et al., CQG submitted to, proceeding of GWDAW11 (2007).9. S. Barthelmy et al., GRB Coordinates Network, 3982 (2005).10. D. Grupe et al., GRB Coordinates Network, 3983 (2005).11. http://www.astro.ku.dk/p˜allja/GRBsample.html12. S. Kobayashi and P. Me´sza´ros, ApJ, 589, 861 (2003).13. E. Cuoco, VIR-NOT-EGO-1390-305 & VIR-NOT-EGO-1390-308 (2005).14. E. Cuoco et al., VIRGO NOTE, in preparation (2007).

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