Di Girolamo, T

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29th International Cosmic Ray Conference Pune (2005) 4, 431434Performance of the Argo-YBJ detector operated in scaler modeT. Di Girolamo , G. Di Sciascio ,H. H. He,I. James, P. Salvini,P. Vallania, Y. G. Wangand F. R. Zhufor the ARGO-YBJ Collaboration(a) INFN, Napoli, Italy(b) IHEP-Beijing, China(c) University of Pavia and INFN, Pavia, Italy(d) INAF-IFSI and INFN, Torino, ItalyPresenter: F. R. Zhu (zhufr@ihep.ac.cn), chn-zhu-F-abs1-og24-oralThe ARGO-YBJ experiment in Tibet has been designed to decrease the energy threshold of typical EAS arraysby exploiting the high altitude location (4300 m a.s.l.) and the full coverage (92 %). The lower energy limitof the detector (E  1 GeV) is reached by the single particle technique, recording the counting rate at fixedtime intervals. At these energies, local (e.g. solar GLE) and cosmological (e.g. GRBs) effects are expectedas a significant enhancement of the counting rate over the background. Results on the search of GRBs incoincidence with Swift, HETE and Integral satellites are presented.1. IntroductionA real snapback in the GRB understanding was given by the discovery, due to the Beppo-SAX satellite ex-periment, of the associated afterglows, and the resultant observations in different wavelenghts leading to thedistance determination. Nevertheless, there is still a lot of work to be done, and the study of the spectrumhigh energy tail is an important item. Most GRBs have been observed in the energy range between 20 KeVand 1 MeV. Photons with E  1 GeV have been detected only for 3 GRBs, by EGRET, that gave in any casethe fundamental result of the existence in their flux of photons with energy up to 18 GeV (GRB940217). Atthese energies the detection from space is hampered by the very low fluxes, requiring large collection areas;from ground the relevant energy range 1 GeV - 1 TeV is partly covered by the Cˇerenkov telescopes, which lastgeneration has been designed to point at the detected GRB direction in a very short time. The duty cycle ishowever very low: a wide field detector operating continuously is necessary. The traditional EAS arrays havea threshold energy (  50 TeV) too high, in a region where a signal is strongly absorbed during the propagationdue to the  -  interaction also for moderate distances. A large sensitive area at very high altitude is a verygood solution: if operated in scaler mode, i.e. counting the number of particles crossing the detector in agiven time interval and searching for an excess in coincidence with a detected signal, it can cover the relevantenergy range [1]. Moreover, this detector can be used for GLE studies, usually associated to solar flares, incoincidence with neutron monitors and satellite proton flux monitors. The limitation of this technique is thenecessity of working in coincidence, and after the CGRO death no GRB wide field monitor was in operationfor years; now, waiting for GLAST, Swift is doing an excellent work, flanked by Integral and HETE.2. The DetectorThe experiment is located in Tibet (4300 m a.s.l.) in a huge building (100x110 m  ) instrumented with a singlelayer of Resistive Plate Counters (RPC)[2]. At the end of phase 1 (first 2006), 5600 m  of active detector areexpected; 1900 m  are in operation at this moment. The detector has a modular structure, the basic modulebeing a cluster (5.7 x 7.6 m  ), divided into 12 chambers. Each chamber is read as 10 independent pads of56 x 62 cm  , that are individually acquired and that represent the high granularity pixel of the detector. Thecarpet is connected to 2 different DAQs, working independently, corresponding to shower mode and scaler432 T. Di Girolamo et almode operations. In shower mode, for each event the location and timing of each detected particle is recorded,allowing the lateral distribution and arrival direction reconstruction; in scaler mode, only the counting rate ofeach cluster is measured every 0.5 s, with no informations on the arrival direction and spatial distribution ofthe detected particles. For each cluster, 4 scalers are used to record the  1,  2,  3 and  4 rates, with thecoincidences defined in a narrow time window (150 ns) and counting rates respectively  40 KHz,  2 KHz, 300 Hz and  120 Hz. The counting rates for a given multiplicipity are then obtained using the relation:n 	 = n 	 - n 	 . Using 4 scalers instead of 1 doesn’t improve significantly the sensitivity, but allows us togive an indication of the source spectrum in case of positive detection. For scaler mode operation, the carpethas worked from October 1st, 2004 to March 10th, 2005 with 667 m  sensitive area and 79% efficiency;since March 17th, 2005, 1750 m  are in operation with 88% efficiency (updated June, 2005). The data arecontinuously checked to give an effective monitoring of the detector performances.3. Effective Area calculationA detailed MC simulation of both EAS development in the atmosphere and detector response has been car-ried out for protons and photons of fixed energy ranging from 1 GeV to 1 TeV and zenith angle of 0  , 20 and 40  . The CORSIKA code 6.18 [3] with QGSJET hadronic interaction model has been used with a fullelectromagnetic component development down to E  = 1 MeV for both electrons and  ’s. For our purposesit is of crucial importance to correctly simulate the cluster size, i.e. the correspondance between the numberof particles hitting the detector and the number of signals generated on the different multiplicity channels. Forthis reason, the detector simulation results have been compared with an analitical calculation of the multiplicityspread based on the measured cluster size.10-410-310-210-111010 210 310 410 51 10 102103E (GeV)Effective Area (m2 )10-610-510-410-310-210-111010 210 310 410 51 10 102103E (GeV)Effective Area (m2 )Figure 1. Effective area for primary photons (a) and protons (b) with zenith angle  = 20  . The curves refer to the differentmultiplicity channels: n=1 (continuous), n=2 (dashed), n=3 (dotted) and n  4 (dot-dashed).Fig. 1 (a,b) shows the effective area for inclined (  = 20  ) photons and protons in the 4 multiplicity channels.The values for protons are lower than that for photons at the same energy, increasing the sensitivity of themeasurement; moreover the geomagnetic cutoff at YBJ is  11 GV (depending on the arrival direction, bothzenith and azimuth), giving an additional bonus to the measurement sensitivity. The effective area is thenconvoluted with the primary spectrum, with spectral indexes of the differential power law spectra  = -2 forphotons (up to E ﬁﬀ  = 100 GeV) and  = -2.7 for protons, taking into account in this case the geomagneticPerformance of the Argo-YBJ detector... 433Table 1. Mode energy and energy range (full width half maximum) for the 4 multiplicity channels, obtained from theconvolution between the effective area and the primary spectrum, for ﬂ ’s and protons.Multiplicity E ﬃ "!$#" (GeV) E %'&$(*)$#+ (GeV) E ﬃ "!$#-, (GeV) E %'&$(*)$#., (GeV)1 5 1-83 6.5 1.8-332 16 1.2-100 11 2.8-653 25 2-100 25 6.4-123 4 100 19-100 52 12.5-240bending [4]. The mode of the  distributions increases from E = 5 GeV for singles (n=1) to E = 100 GeV forn  4 with a broad distribution, giving the energy region of interest for the detector. Table 1 shows the modeenergy and the energy range for the 4 multiplicity channels.4. ResultsTypical fluence upper limits are obtained using the experimental counting rates and a GRB spectrum model.Assuming a power law spectrum up to E ﬁﬀ  = 100 GeV with differential spectral index  = -2, zenith distance = 20  , time duration / t = 10 s and statistical significance k = 3, the fluence upper limits shown in fig. 2 areobtained for the 4 multiplicity channels. The GRB is assumed close enough to ignore the  absorption (pair10-610-510-410-310-210-11 10 102E(GeV)Fluence (erg/cm2 )Figure 2. Typical Argo-YBJ uence upper limits for4 multiplicity channels compared with other experiments(see text).10-610-510-410-310-210-11 10 102E(GeV)Fluence (erg/cm2 )Figure 3. Fluence upper limits for 4 GRBs in the Argo-YBJ f.o.v. ( 10 40). For GRB050408 only, the ﬂ absorp-tion is considered (z=1.24).creation on stellar light photons). In the same plot, some fluence upper limits obtained by other experiments[5],[6] are reported for comparison (scaled with 23/ t when necessary). Notice from fig. 2 that if the 4 fluenceupper limits are connected by a line, the corresponding differential energy spectrum index is  = -1.8: a goodupper limit can be derived also in case of positive detection in the lower energy channel only (n = 1). The GRBsearch is done in coincidence with Swift, HETE and Integral satellite experiments. The ARGO-YBJ experiment434 T. Di Girolamo et alis linked to the Swift Follow-Up project, allowing a complete GRB analysis in “real time”, while for the other2 satellites the information is collected on the web [7]. The GRB search was started in correspondance withthe first GRB detected by Swift on Dec 17, 2004. Up to now (June 2005) 49 GRBs have been detected, mostlyby Swift; 4 of them are in the field of view of Argo ( 54 40  ). No signal evidence has been detected; fig. 3shows the fluence upper limits for these events, using the measured low energy time duration and spectral index(when available, otherwise we assumed  = -2), E ﬃ &$6 = 100 GeV and k = 3. For GRB050408 the distance isknown (z = 1.24) and the upper limit is calculated including a simple  absorption model [8]; for the others z= 0 is assumed. Apart from GRBs, 2 other events have been studied: the SGR 1806-20 flare on Dec 27, 2004,under the ARGO horizon (  = 99.4  ) but perturbing the whole ionosphere, and the solar flare (Class X7) of Jan20, 2005, connected to a GLE and detected by the Milagro scaler system [9]. No significant coincident excesshas been detected so far.5. ConclusionsThe search for GRBs chosen with 734 40  in the Argo-YBJ field of view has given no positive result. Never-theless this simple technique, due to the large sensitive area at very high mountain location, has already showna good sensitivity, with typical fluence upper limits of 10 89 - 10 8;: erg/cm  in the 1-100 GeV energy range.Moreover, using 4 counting channels corresponding to 4 energies well covering the 1-100 GeV region, we cangive an indication on the GRB spectrum slope in case of positive detection, and even with a significant excessin the most sensitive multiplicity channel only, a valid maximum spectral index can be derived. We expectto increase the sensitivity by a factor  2 with the detector completion, and to have an equivalent improve-ment converting the secondary photons of the showers with a 0.5 cm thick layer of lead. A preliminary study[10], applied to the whole Argo-YBJ, has shown that if the GRB spectrum extends at least up to 100 GeV forthe most energetics events and their distance doesn’t exceed z = 2, we expect to detect  1 event/year. Withthese expectations, an accurate check of the detector stability and performances is of crucial importance; apreliminary work can be found in [11].6. AcknowledgementsThis work is supported in part by NSFC(10120130794), the Chinese Ministry of Science and Technology, theChinese Academy of Sciences, the Key Laboratory Astrophysics, IHEP, C.A.S and INFN, ItalyReferences[1] S. Vernetto, Astrop. Phys. 13, 75 (2000).[2] Z. Cao et al., ”Status of the ARGO-YBJ experiment”, these proceedings.[3] D. Heck et al., Karlsruhe Report FZKA 6019 (1998).[4] T.K. Gaisser and M. Honda, Ann. Rev. Nucl. Part. Sci. 52, 153 (2002).[5] M. Aglietta et al., Astroph. Journal 469, 305 (1996).[6] A. Castellina et al., Proc. 27th ICRC, Hamburg (2001), 2735.[7] J. Greiner 2005 GRB webpage, http://www.mpe.mpg.de/ jcg/grbgen.html[8] M. H. Salamon and F. W. Stecker, Astroph. Journal 493, 547 (1998).[9] G. Sinnis, Presentation at the Conference “Towards a Network of Atmospheric Cherenkov Detectors”,Palaiseau (France), 27-29 April 2005.[10] A. Aloisio et al., Il Nuovo Cimento 24C, 4-5, 739 (2001).[11] P.Vallania et al., Proc. 28th ICRC, Tsukuba (2003), 2761.

Performance of the Argo-YBJ detector operated in scaler mode

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