We report an update of the search for emission from Gamma Ray Bursts (GRBs) in the energy range 1−100 GeV in coincidence with the prompt emission detected by satellites, using the large field of view (about 2 sr) air shower detector ARGO-YBJ, appropriate to monitor unpredictable and short duration events like GRBs. The search has been carried out using the single particle technique in time coincidence with satellite detections both for single events and by stacking GRBs in time and phase. Between December 2004 and April 2011, 131 GRBs detected by different satellites occurred in the ARGO-YBJ field of view (zenith angle within 45 degrees). For 110 of these we searched for a high
energy counterpart in the ARGO-YBJ data, finding no statistically significant signal. The resulting fluence upper limits between 1 and 100 GeV reach values as low as 10^(−5) erg/cm2, and in one case (GRB090902B) can be compared with observations by the LAT instrument on the Fermi satellite

C. Vigorito

P. Vallania

T. Di Girolamo

Archivio della ricerca - Università degli studi di Napoli Federico II

32ND INTERNATIONAL COSMIC RAY CONFERENCE, BEIJING 2011Update of the Search for Gamma Ray Bursts with ARGO-YBJ in Scaler ModeT. DI GIROLAMO1 , P. VALLANIA2 , C. VIGORITO3 FOR THE ARGO-YBJ COLLABORATION1Universita` di Napoli “Federico II” and INFN-Napoli, Italy2IFSI-INAF and INFN-Torino, Italy3Universita` di Torino and INFN-Torino, Italytristano@na.infn.itAbstract: We report an update of the search for emission from Gamma Ray Bursts (GRBs) in the energy range 1−100GeV in coincidence with the prompt emission detected by satellites, using the large field of view (about 2 sr) air showerdetector ARGO-YBJ, appropriate to monitor unpredictable and short duration events like GRBs. The search has beencarried out using the single particle technique in time coincidence with satellite detections both for single events and bystacking GRBs in time and phase. Between December 2004 and April 2011, 131 GRBs detected by different satellitesoccurred in the ARGO-YBJ field of view (zenith angle within 45 degrees). For 110 of these we searched for a highenergy counterpart in the ARGO-YBJ data, finding no statistically significant signal. The resulting fluence upper limitsbetween 1 and 100 GeV reach values as low as 10−5erg/cm2, and in one case (GRB090902B) can be compared withobservations by the LAT instrument on the Fermi satellite.Keywords: gamma ray bursts - gamma ray observations - extensive air showers1 IntroductionThe study of GRBs has been carried out mainly from spacedetecting directly the primary photons. Due to the fast de-crease of their spectrum, the operating energies are usu-ally in the keV−MeV range, and only the EGRET instru-ment in the past and now AGILE and the Fermi GammaRay Space Telescope reached the GeV region, with maxi-mum detectable energies of 30 and 300 GeV, respectively.At ground level, the search can be performed by means oflarge area extensive air shower detectors operating at highaltitude, measuring the secondary particles produced by theinteraction of the primary photons with the atmospheric nu-clei. This search, started many years ago (see for example[1, 2, 3, 4, 5]) as a particular way to use experiments de-signed for gamma-ray astronomy, requires very stable andreliable detectors. Moreover, at lower energies the numberof secondary particles reaching the ground, often only one,does not allow the measurement of the arrival direction,making unfeasible an independent detection.More than forty years after their discovery and more thanten years after the detection of the first afterglow by Bep-poSAX, the physical origin of the GRBs is still under de-bate, allowing a variety of theoretical models. In these con-ditions, and mainly at energies >1 GeV, any further resultcould be of great importance to improve the knowledge ofthe GRB puzzle. ARGO−YBJ may reveal the spectral cut-off of GRBs and constrain their emission models.2 The DetectorThe ARGO−YBJ experiment is located at the YangBa-Jing Cosmic Ray Laboratory (Tibet, P.R. China, 30.11◦N,90.53◦E, 4300 m a.s.l., atmospheric depth 606 g/cm2).The detector is constituted by a central carpet large∼74×78 m2, made of a single layer of Resistive PlateCounters (RPCs), operated in streamer mode, with 93% ofactive area and enclosed by a guard ring partially (∼20%)instrumented up to ∼100×110 m2. The apparatus has amodular structure, the basic data acquisition element beinga “cluster” (5.7×7.6 m2), made of 12 RPCs (2.85×1.23m2 each). Each chamber is read by 80 external strips of6.75×61.80 cm2 (the spatial pixels), logically organized in10 independent pads of 55.6×61.8 cm2, which representthe time pixels of the detector.The detector is connected to two independent data ac-quisition systems, corresponding to operations in “showermode” and “scaler mode”. In shower mode the arrival timeand the location of each particle are recorded using thepads, allowing the detailed reconstruction of the showerlateral distribution and of the arrival direction. In scalermode the total counts of each cluster are obtained addingup the signal coming from its 120 pads. This signal is putin coincidence in a narrow time window (150 ns) and readby four independent scaler channels, measuring the totalcounting rate of≥1,≥2,≥3 and≥4 pads fired every 0.5 s.The corresponding measured rates are ∼40 kHz, ∼2 kHz,∼300 Hz and ∼120 Hz, respectively. This technique doesDI GIROLAMO et al. SEARCH FOR GRBS WITH ARGO-YBJnot allow the measurement of the energy and arrival direc-tion of the primary gamma rays (the field of view is about2 sr, only limited by the atmospheric absorption), but theenergy threshold can be pushed down to ∼1 GeV, overlap-ping the highest energies investigated by satellite experi-ments. Moreover, with four measurement channels sensi-tive to different energies, in case of positive detection valu-able information on the high energy spectrum slope andpossible cutoff may be obtained [6].Since for the GRB search in scaler mode the authenticationis only given by the satellite detection, the stability of thedetector has to be deeply investigated. Details of this study,together with the determination of the effective area, upperlimit calculation and expected sensitivity, can be found in[7].3 Search for emission from GRBsThe present update collects data from November 2004 (cor-responding to the Swift satellite launch) to April 2011, witha detector active area increasing from ∼700 to ∼6600 m2.During this period, a total of 131 GRBs, selected from theGCN Circulars Archive1, was inside the ARGO-YBJ fieldof view (i.e. with zenith angle θ ≤ 45◦); for 110 of thesedata were available and have been investigated by search-ing for a statistically significant excess in the counting ratescoincident with the satellite detection. In order to extractthe maximum information from the data, two GRB analy-ses have been implemented:• search for a signal from every single GRB;• search for a signal from stacked GRBs.For both analyses, the first step is the data cleaning andcheck. For each event, the Poissonian behaviour of thecounting rates for the four multiplicity channels (≥1, ≥2,≥3,≥4) for all the clusters is checked using the normalizedfluctuation function:f = (s− b)/σ, σ =√b+ b/20 (1)in a period of ±12 h around the GRB trigger time. Inthis formula, s is the number of counts in a time intervalof 10 s, b the number of counts in 10 s averaged over atime interval of 100 s before and after the signal, and σthe standard deviation, with about 400 independent sam-ples per distribution. The interval of 10 s has been chosento avoid any systematic effect due to environment and in-strument (such as atmospheric pressure and detector tem-perature variations). The expected distribution of f is thestandard normal function; the clusters giving a distributionwith measured σf > 1.2 or with anomalous excesses (i.e.number of entries > 2%) in the tail f > 3 in at least onemultiplicity channel, are discarded. This guarantees thatthe data fulfil the requirements on stability and reliabilityof the detector.)σSignificance (-5 -4 -3 -2 -1 0 1 2 3 4 5Number of GRBs0510152025303540 0.12± = -0.02fit>σ< 0.09± =  1.19fits.d.Figure 1: Distribution of the statistical significances of the110 GRBs with respect to background fluctuations, com-pared with a Gaussian fit.3.1 Search for single GRBsThe counting rates of the clusters surviving the quality cuts(∼92%) are added up and the normalized fluctuation func-tionf ′ = (s′ − b′)/σ′, σ′ =√b′ + b′∆t90[s]600(2)is used to give the significance of the coincident on-sourcecounts. In this formula, s′ is the total number of counts inthe ∆t90 time window given by the satellite detector (cor-responding to the detection of 90% of the photons) and b′is the number of counts in a fixed time interval of 300 s be-fore and after the signal, normalized to the ∆t90 time. Dueto the correlation between different clusters (given by theair shower lateral distribution), the distributions of the sumof the counts are larger than Poissonian. Taking this intoaccount, the statistical significance of the on-source countsover the background is obtained again in an interval of ±12h around the GRB trigger time, using equation (17) of [8](for more details see [7]). All the results are obtained usingthe single particle counting rate C1 = C≥1 − C≥2, whichcorresponds to the minimum primary energy in the ARGO-YBJ scaler mode. Figure 1 shows the distribution of thesignificances for the whole set of 110 GRBs compared witha Gaussian fit, which has mean value −0.02 ± 0.12 andstandard deviation 1.19 ± 0.09. No significant excess isshown, and the maximum significance 3.52σ is obtainedfor GRB080727C, with a chance probability of 2.4% tak-ing into account the total number of GRBs analyzed.Lacking a positive signal, upper limits to fluence are ob-tained in the 1-100 GeV energy range adopting a power1. http://gcn.gsfc.nasa.gov/gcn3 archive.html32ND INTERNATIONAL COSMIC RAY CONFERENCE, BEIJING 2011Zenith angle (deg)0 10 20 30 40 50)2Fluence uppper limit (erg/cm-610-510-410-310-210Figure 2: Fluence upper limits as a function of the zenithangle in the 1-100 GeV range, obtained extrapolating themeasured keV-MeV spectra. Triangles represent GRBswith measured redshift, circles GRBs assumed at z=1.law spectrum and by considering the maximum number ofcounts at 99% confidence level (c.l.), following equation(6) in [9]. For this calculation, two different assumptionsare used for the power law spectrum: a) extrapolation ofthe keV-MeV spectrum measured by the satellite experi-ments, when available; b) a differential spectral index α =-2.5. Since the mean value of spectral indexes measured byEGRET in the GeV energy region is α = -2.0 [10], we ex-pect that the true upper limits lie between these two values.For GRBs with measured redshift, an exponential cutoff inthe spectrum is considered to take into account the effectof extragalactic absorption, which is calculated using thevalues given in [11]. When the redshift is not measured,a value z = 1 is adopted. For the subset of 82 GRBs withspectral index α measured by satellites, the fluence upperlimits have been calculated according to assumption a) andthey are shown in figure 2 as a function of the zenith angle.For the subset of 18 GRBs with measured redshift, the flu-ence upper limits for the two assumed spectra are shownin figure 3. Since the measured low energy differentialspectral indexes for these GRBs are always greater than-2.5, the higher upper limits refer to this extrapolation.For 3 GRBs the measured low energy spectrum is a cutoffpower law, thus only the value obtained assuming α=-2.5is shown. For the other GRBs the rectangles indicate theupper limit range corresponding to differential spectral in-dexes between the fixed value α=-2.5 and the low energymeasurement. For GRB090902B the resulting upper limitrange can be compared with observations by the LAT in-strument on the Fermi satellite (circle in figure 3).When using as the GRB spectrum the extrapolation of theindex measured by satellite experiments, an upper limit toGRB Redshift (z)0 2 4 6)2Fluence uppper limit (erg/cm-510-410-310-210Figure 3: Fluence upper limits of GRBs as a function ofredshift. The rectangles represent the values obtained withdifferential spectral indexes ranging from α=-2.5 to thesatellite measurement. The 3 arrows give the upper lim-its for the former case only (for GRBs with cutoff powerlaw spectrum). The circle shows the integral fluence in the1-100 GeV range for GRB090902B as observed by LAT.the cutoff energy can be determined at least for some GRBsexploiting the ARGO−YBJ scaler mode data, with the fol-lowing procedure. The extrapolated fluence is plotted to-gether with the fluence upper limit as a function of the cut-off energy Ecut. If the two curves cross in the 2-100 GeVenergy range, the intersection gives the upper limit to thecutoff energy. For these GRBs we can state, with a 99%c.l., that their spectra do not extend beyond this value ofEcut, if the slope remains constant. Figure 4 shows the re-sulting cutoff energy upper limits as a function of the spec-tral index for the 19 GRBs with critical intersection in thequoted energy range. For 4 of these (triangles in figure 4)the measurement of the redshift allows the estimation ofthe extragalactic absorption. When the GRB redshift is notavailable, the value z=1 is adopted.3.2 Stacked analysisThe search for cumulative effects by stacking all the GRBseither in fixed time durations or in phases of ∆t90 could en-hance a possible signal, making it statistically significant,even if the emission of each GRB is below the sensitiv-ity of the ARGO−YBJ detector. In this case, less infor-mation could be given compared to the single GRB coin-cident detection, however we must consider that with thestacked analysis we increase the sensitivity by increasingthe number of GRBs, while with the single GRB search weDI GIROLAMO et al. SEARCH FOR GRBS WITH ARGO-YBJGRB spectral index1 1.2 1.4 1.6 1.8 upper limit (GeV)cutE110210Figure 4: Upper limits to the cutoff energy of GRBs asa function of the spectral index obtained extrapolating thekeV-MeV spectra measured by satellites. The triangles rep-resent the values obtained taking into account the extra-galactic absorption for the GRBs with measured redshift,for the others z=1 is adopted.of trials. The stacked analysis is carried out supposing acommon timing feature in all GRBs.First, the total number of counts in a time window ∆t (with∆t=0.5, 1, 2, 5, 10, 20, 50, 100, 200 s) after T0 (the trig-ger time given by the satellites) for all the GRBs are addedup. This is done in order to search for a possible cumula-tive high energy emission with a fixed duration after T0. Apositive observation at a fixed ∆t could be used as an alter-native value to the standard ∆t90 duration. Indications ofa delayed high energy component have been observed bysatellite experiments in some GRBs. The resulting signifi-cances for the 9 time bins give no evidence of emission fora certain ∆t. Since the bins are not independent, the distri-bution of the significances of the 9 time bins is comparedwith random distributions obtained for starting times dif-ferent from T0 in a time window of ±12 h around the trueGRB trigger time. The resulting overall significance of theGRBs stacked in time with respect to random fluctuationsis -0.57σ.A second search is carried out to test the hypothesis thatthe high energy emission occurs at a particular phase of thelow energy burst, independently of the GRB duration. Forthis study, the 92 GRBs with ∆t90 ≥ 5 s (i.e. belongingto the “long GRB” population, defined by ∆t90 ≥ 2 s)have been added up in phase after scaling their duration.This choice has been made for both physical and technicalreasons, adding up the counts for GRBs of the same classand long enough to allow a phase plot with 10 bins giventhe time resolution of 0.5 s. No evidence of emission ata certain phase is obtained, and the overall significance ofthe GRBs stacked in phase (obtained adding up all the bins)with respect to background fluctuations is -0.73σ.3.3 ConclusionsIn this paper we have reported a study concerning thesearch for GeV photons from 110 GRBs carried out by theARGO−YBJ air shower detector operated in scaler mode.In the search for GeV gamma rays in coincidence with thelower energy GRBs detected by satellites, no evidence ofemission was found for any event. The search for a signalfrom GRBs stacked both in time and in phase has shownno deviation from the statistical expectations, therefore ex-cluding any integral effect. The obtained upper limits to thefluence in the 1-100 GeV energy range depend on zenithangle, time duration and spectral index, reaching values aslow as ≈ 10−5erg/cm2, in the case of GRB090902B avalue just a factor about 3 higher than the fluence extrap-olated from LAT observations. Since upper limits to thecutoff energy can be set for several GRBs between 2 and100 GeV, we confirm that the simple extrapolation of thepower law spectrum measured at low energies is not alwayspossible [12].References[1] O’Brien S., Porter N.A., Astrophysics and Space Sci-ence, 1976, 42: 73-76[2] Morello C., Periale L., Navarra G., Il Nuovo CimentoC, 1984, 7: 682-688[3] Alexandreas D.E. et al., The Astrophysical Journal,1994, 426: L1-L3[4] Castellina A. et al., Proceedings of the 27th ICRC,2001, 2735-2738[5] Aglietta M. et al., The Astrophysical Journal, 1996,469: 305-310[6] Aielli G. et al., The Astrophysical Journal, 2009, 699:1281-1287[7] Aielli G. et al., Astroparticle Physics, 2008, 30: 85-95[8] Li T., Ma Y., The Astrophysical Journal, 1983, 272:317-324[9] Helene O., Nuclear Instruments and Methods, 1983,212: 319-322[10] Dingus B.L., Catelli J.R., Schneid E.J., Proceedingsof the 25th ICRC, 1997, 3: 29-32[11] Kneiske T.M. et al., Astronomy and Astrophysics,2004, 413: 807-815[12] D. 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Update of the search for Gamma Ray Bursts with ARGO-YBJ in scaler mode

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