When the data acquisition (DAQ) system of an underwater neutrino telescope is interfaced with IBAS and GCN, all raw data can be saved to disk for a few minutes when a GRB alert message is received. If the DAQ system is designed such that it constantly buffers a few minutes of raw data, the data that were taken before the alert could then also be saved to disk. Consequently all raw data covering a few burst durations before and after the burst will be available. Any time-correlated neutrino signal will thus be included in these data. The data can be analysed offline using the known position of the GRB. Using the time and direction information of GRBs, the detection threshold for the neutrinos they emit could be lowered. The increase in detection efficiency achieved in this way will be quantified

for the ANTARES experiment

Bouwhuis, M.

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DOI 10.1393/ncc/i2005-10158-5IL NUOVO CIMENTO Vol. 28 C, N. 4-5 Luglio-Ottobre 2005Increasing the detection efficiency for neutrinos from GRBs(∗)M. Bouwhuis on behalf of the ANTARES CollaborationNIKHEF - Kruislaan 409, 1098 SJ, Amsterdam, The Netherlands(ricevuto il 23 Maggio 2005; pubblicato online il 28 Ottobre 2005)Summary. — When the data acquisition (DAQ) system of an underwater neutrinotelescope is interfaced with IBAS and GCN, all raw data can be saved to diskfor a few minutes when a GRB alert message is received. If the DAQ system isdesigned such that it constantly buffers a few minutes of raw data, the data thatwere taken before the alert could then also be saved to disk. Consequently all rawdata covering a few burst durations before and after the burst will be available. Anytime-correlated neutrino signal will thus be included in these data. The data can beanalysed offline using the known position of the GRB. Using the time and directioninformation of GRBs, the detection threshold for the neutrinos they emit could belowered. The increase in detection efficiency achieved in this way will be quantifiedfor the ANTARES experiment.PACS 98.70.Rz – γ-ray sources; γ-ray bursts.PACS 96.40.Tv – Cosmic ray neutrinos and muons.PACS 01.30.Cc – Conference proceedings.1. – IntroductionNeutrinos from GRBs can be detected with neutrino telescopes, situated under wateror in ice. The detection principle of these detectors is based on the detection of theCherenkov light emitted by the lepton that is produced after a charged current neutrinointeraction near the detector, and that moves relativistically through the water or ice.This light can be detected by photo multiplier tubes (PMTs) that are arranged in a three-dimensional array in the detection medium. Muon neutrinos are mainly considered, asthe muon resulting from the interaction can travel large distances. From the positionsof the PMTs and the arrival times of the detected photons the path of the muon can bereconstructed and its direction can be derived. From its direction can be concluded ifit concerns a neutrino from a GRB, as the direction of the muon is approximately thesame as that of the neutrino.The neutrino telescope considered here is the ANTARES detector [1]. The construc-tion of the detector has started, and the full detector will be completed in 2007.(∗) Paper presented at the “4th Workshop on Gamma-Ray Burst in the Afterglow Era”, Rome,October 18-22, 2004.c© Societa` Italiana di Fisica 805806 M. BOUWHUIS ON BEHALF OF ANTARES COLLABORATION2. – Standard data taking in ANTARESThe data acquisition (DAQ) system of the ANTARES detector, that takes care ofthe readout of the detector, the transportation of the data from the detector to shore,and the processing of the data, has some unique features. In this system all raw dataare sent to shore. On shore the data arrive in a computer farm consisting of a few tensof computers. Each of these PCs runs a filter program that looks for correlations in thedata. Only the data that resemble the data produced by physics signals are stored ondisk. All other data are considered as background. The physics signal stored on disk isused for the physics analyses. Not all data can be used for the physics analysis as thedata output of the detector consists mainly of background. The background is partlycaused by atmospheric muons and neutrinos produced after the interaction of cosmic raysentering the atmosphere. Most of the background is due to bioluminescence and radioactive decay of nuclei in the sea water, and produces, in contrast to the atmosphericbackground, uncorrelated data. As a result of this, the total data rate is about 1 GB/ s.The data-processing programs have to filter the data in real time as the detector willcontinuously take data, 24 hours per day. The resulting data rate after filtering is lessthan 1 MB/ s, which is manageable for the data analysis. Because of the required speedof the algorithms used for the processing of the data, cuts are applied that imply partialloss of physics signal.3. – Data taking in case of a GRBThe DAQ system has a socket connection with the GCN [2] and IBAS [3] warningsystems for GRBs. When a GRB alert message is received from either of these systems,a possible neutrino signal from this GRB could be detected at the same time. Whenthis happens, all raw data are saved to disk as shown in fig. 1, instead of processing thedata online. This will continue for a few minutes, as GRBs can last up to a few tens ofseconds, and the neutrino signal from a GRB is expected to arrive within a few burstdurations around the burst. This way of data taking will only last for a few minutesbecause of the high data rate. After this time, the system will return to normal datataking. Later, when the location of the GRB is distributed by the same GRB warningsystems, a specialised analysis can be applied to these data. The fact that a GRB lastsfor only a short time makes it possible to handle all these data. The standard dataprocessing and analysis tools, that are normally used, look for a neutrino signal in alldirections. In this case the location of the GRB and thus the direction of the neutrinosis known. This information is used to analyse the data.Not only the data taken during these few minutes will be available for the dataanalysis, but also the data that were in the memories of the data processing PCs at thetime the alert message was received. The data processing programs hold continuously asmuch data as possible in memory before processing them. With tens of data processingPCs, all raw data corresponding to about one minute of data taking are buffered. Thesedata will also be saved to disk when an alert message is received. The buffered data willcover the delays involved in the GRB warning systems, and a possible early neutrinosignal [4]. In this way, all data that were taken during a few burst durations around theburst are available and will contain any time-correlated neutrino signal, assuming thatthe alert message arrives within about 20 seconds after the detection of the gamma rays.INCREASING THE DETECTION EFFICIENCY FOR NEUTRINOS FROM GRBS 807alert triggerDataFilterunfiltered dataall dataalert messageposition messagedetectorGCNIBASspecial analysisthat uses direction informationFig. 1. – Special data taking when a GRB is detected by using the GCN and IBAS systems asexternal triggers. Normally all data are sent from the detector to shore, and processed thereby the DataFilter processes. When a GRB alert message is received, indicated by the zig-zagarrows, the DataFilter programs are instructed to stop filtering the data. Instead, all data arewritten to disk for a few minutes. Later, when the position of the GRB is received, indicatedby the wavy arrows, the data will be analysed using the direction information.10-510-410-310-210-1110102 103 104 105 106 107instrumented volumespecial data taking, analysis using direction informationnormal data taking, standard analysisE (GeV)effective volume (km3)Fig. 2. – The effective volume as a function of the neutrino energy for neutrinos from GRB030329when the special data taking is applied, and the special analysis was used for the data that weretaken during this burst. Also shown is the effective volume for normal data taking and standardanalysis (dashed line). The dotted line indicates the instrumented volume of the detector.808 M. BOUWHUIS ON BEHALF OF ANTARES COLLABORATION4. – ResultsAs soon as the message with the location of the GRB has arrived, the stored raw dataare analysed using specialised algorithms that use this direction information. Becauseof this extra information, and because the data processing is not done in real time, lessstrict cuts are applied to the data.This results in a higher efficiency for the physics signal. Considering the expectedfrequency of detection of GRBs by satellites, and the time needed to analyse the datain this way, the message with the location of the GRB should arrive within 12 hoursafter the initial alert message. The described way of data taking and data analysis hasbeen applied to simulated data. For this, GRB030329 was used as an example. Theeffective volume that can be obtained with the ANTARES detector for neutrinos fromthe direction of this GRB is shown in fig. 2 as a function of the neutrino energy. Theeffective volume is 2-3 times bigger than when no special data taking and data analysiswas applied.5. – ConclusionsNeutrino telescopes can profit from the GRB warning systems as the moment a GRBoccurs and the direction of the neutrinos they emit is known. The alert messages shouldarrive within about 20 seconds after the gamma rays are detected, and the final messagewith the position of the GRB within 12 hours. These GRB warning systems are especiallyof great importance for neutrino telescopes like ANTARES, that have the all-data-to-shore concept implemented, where all raw data covering a few minutes can be saved,including data that were taken prior to the arrival of the alert message. Using the timeand direction information obtained from the GRB warning systems in this way increasesthe discovery potential of neutrinos from GRBs. Considering the expected number ofat least 100 GRB triggers from GRB satellites per year [5], the 50% field of view of theANTARES detector, and the predicted neutrino fluxes from GRBs, this could result inthe detection of a few events per year [6].REFERENCES[1] Moscoso L., 32nd International Conference on High Energy Physics, Beijing, edited by H.Chen, D. Du, W. Li and C. Lu (World Scientific, Singapore) 2005, p. 444.[2] Barthelmy S., Proceedings of the 5th Huntsville GRB Workshop, AIP Conf. Ser., 526(2000) 731.[3] Mereghetti S. et al., A&A, 411 (2003) L291.[4] Razzaque S. et al., Phys. Rev., D69 (2004) 023001.[5] Gehrels N. et al., ApJ, 611 (2004) 1005.[6] Bouwhuis M., PhD thesis, University of Amsterdam (2005) http://www.nikhef.nl/pub/services/newbiblis/theses.php.

Increasing the detection efficiency for neutrinos from GRBs

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Increasing the detection efficiency for neutrinos from GRBs

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