Vernetto, S

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29th International Cosmic Ray Conference Pune (2005) 4, 375378Sky survey with the ARGO-YBJ detectorS.Vernetto , C. Bleve, S.W.Cui, T.Di Girolamoand X.X Zhoufor the ARGO-YBJ Collaboration(a) IFSI-Istituto Nazionale di Astrosica and INFN, Torino (Italy)(b) Dipartimento di Fisica Universita’ di Lecce and INFN, Lecce (Italy)(c) Physics department, He Bei Normal University, Shijiazhuang (China)(d) INFN, Napoli (Italy)(e) Inst.of Modern Physics, South West Jiao Tong University, Chengdu (China)Presenter: S.Vernetto(vernetto@to.infn.it), ita-vernetto-S-abs1-og23-oralThe ARGO-YBJ experiment is planned to detect gamma ray sources with an energy threshold of a few hundredsof GeV. The large field of view of the detector and its high duty cycle allow the continuous monitoring of alarge part of the sky, searching for both unknown stable sources and transient emissions as AGN flares andGamma Ray Bursts. In this work we present the first data obtained with a subset of the detector (covering 1900 m) concerning the search for stable and transient sources in the declination range 	 .1. The ARGO-YBJ experimentARGO-YBJ is a “full coverage” air shower detector under construction at the Yangbajing High Altitude CosmicRay Laboratory (Tibet, China) at 4300 m a.s.l. (lat=30.11  N, long=90.53  E). One of the main goals of theexperiment is the detection of  rays from galactic and extragalactic sources with an energy threshold of a fewhundreds of GeV. The extreme altitude of the detector and the use of a large full coverage layer of counters,allow the detection of showers in the primary energy range typical of the Cherenkov technique. However, unlikeCherenkov telescopes, ARGO-YBJ has a large field of view (  2.2 sr) and a duty cycle  100  , allowing thecontinuous and simultaneous observation of a large fraction of the sky, making thus easier the detection ofpreviously unknown  ray emitters and transient events like Gamma Ray Bursts and AGN flares.ARGO-YBJ consists of a 74  78 m“carpet” realized with a single layer of Resistive Plate Counters (RPCs),surrounded by a partially instrumented “ guard ring”, for a total active area of 6400 m. The detector is dividedinto 18480 basic elements, of dimensions 56  62 cm, the “pads”, providing the space-time pattern of theshower front. The time resolution is  1 ns. The detector will be covered by a 0.5 cm thick layer of lead, inorder to convert a fraction of the secondary gamma rays in charged particles, and to reduce the time spread ofthe shower front, increasing the angular resolution. A detailed description of the experiment is given in [1].A subset of the detector (1900 mof RPCs,  30  of the complete layout) is taking data since December 2004.In this paper we present our first sky survey results searching for stable gamma ray sources and short transientevents as high energy GRBs.2. The dataThe data used in this analysis have been recorded by a detector subset consisting of a  47 x 41 mRPC carpet,without the lead converter layer. The minimum number of fired pads ﬁﬀﬁﬂ required to trigger the detector is 60.This temporary trigger configuration (used to debug the detector) corresponds to an energy threshold relativelyhigh: according to simulations, given a  ray source with a power law spectrum of index ﬃ =2.5 (2.0) extendingup to 50 TeV and zenith angle ! "#$ , the median energy of the detected  rays is  4 (  8) TeV.The arrival direction of the primary particles have been reconstructed by fitting the shower front with a conical376 S.Vernetto et alStandard deviations-5 -4 -3 -2 -1 0 1 2 3 4 5Figure 1. Preliminary sky map obtained in 1.0 10 % hours of data taking.The three small circles indicate the positions of the Crab Nebula, Mrk 421and Mrk 501 (from left to right).Figure 2. Distribution of the excesses in the26670 bins of the map in unit of standard de-viations, with the Gaussian t overlapped.shape of slope 0.03 ns m &(' . The position of the shower core has been calculated by means of the MaximunLikelihood Method applied to the lateral density profile of the shower[2]. According to simulations, the angularresolution depends on the number of fired pads: for !ﬁﬀﬁﬂ*) 60, a circular window around the source with ahalf opening angle +, .-/10 contains  70  of the events induced by a gamma ray of energy 1-10 TeV andzenith angle ! 2#3 .The data set of this analysis has been recorded from 2004 Dec 24 to 2005 Mar 23, for a total run time of 1006.5hours. The event rate is  160 Hz. Since in this work we consider the events with zenith angle 5460#7 , we aremonitoring the declination band 	$89:;3 , corresponding to 8.3 sr (66  of the celestial sphere). Nogamma-hadron discrimination is performed on these preliminary data.3. Search for stable gamma ray sourcesThe search for point gamma ray sources consists in filling a sky map with all the detected showers and com-paring each bin content with the expected background due to cosmic rays. Taking into account the angularresolution of the detector in the 1-10 TeV energy range discussed in the previous section, we adopt squaredbins of size <3=< , i.e. <3 in declination (  ) and <$?>#@BA#C? in right ascension. Since the position of possiblesources is unknown we oversample the sky in order to detect a possible source near the edge of a bin. Every binthus is shifted by -D in both  and ﬃ . Each map contains a total of 26670 non independent bins. The contentof each bin E is compared with the expected background 5F . The background map has been built using amethod similar to the “time swapping method” [3], randomly changing the time of each real event inside a timeinterval of a few hours, during which both the background and the response of the detector can be assumedconstant. Each real event is used to generate 10 different background events, in order to increase the statisticsof the background map.Fig. 1 shows the sky map obtained with the whole set of data, corresponding to 5.7 -GH events. For each binthe value of the variable IKJ= .LMENOFQPR>S F is reported. The distribution of IKJ is well fitted by a Gaussdistribution with T =1.02 (see Fig.2). No excess larger than 4.0 T is observed. Given the limited run time ofthe present data, the Crab Nebula is expected to give a signal of  0.8 standard deviations. Assuming a sourceSky survey with the ARGO-YBJ detector 377Figure 3. GRB sky survey: distribution of the integralprobability obtained in the sky maps of duration UWV =10 s.Figure 4. GRB sky survey: distribution of the integral prob-ability obtained in the sky maps of duration U	V =300 s.with a Crab-like spectrum (i.e. spectral index ﬃ =2.59) at a declination * X< , where ARGO-YBJ is mostsensitive, the gamma ray flux corresponding to 4.0 T is Y[Z\J  5 Crab units. The flux limit increseas as thedeclination moves from 30  , being Y]Z\J  7(  6) Crabs at ^ ;-G3 (50 GP and Y]Z\J  10 (  12) Crabs at _ 2$(60  ).4. Search for transient gamma ray sourcesA very high energy Gamma Ray Burst is observable as a transient excess of showers with arrival directionsconsistent with a point source. In order to single out an excess without knowing the position in the sky, the timeduration nor the occurrence time, for every run we built a set of sky maps (as described in the previous section)each one characterized by a start time `Ra and a time duration b` consistent with the possible GRB duration. Inthis analysis we choose b` = 10, 50, 100 and 300 s. The start time of the cedMf map of duration b!` is definedas `gah 9`ga&i'kjb`R># , where `'correspond to the run start time. In this way the maps oversample the time, inorder to detect more efficiently a GRB with unknown time occurrence. In order to single out possible excesses,each map must be compared with the corresponding “background map” containing the events expected fromthe cosmic ray background. To build the “background” maps of so short time duration we use the followingprocedure. First we build a total map in local equatorial coordinates (declination  and hour angle l ) containingall the events recorded during a time interval of a few hours. The “background” map corresponding to a singlemap of duration b!` and start time ` a , is then obtained transforming the total local map in a celestial coordinatemap (in  and ﬃ ), via the relation ﬃm n` Eoapﬂ ql , where ` Eoapﬂ is the sidereal time corresponding to the map centraltime ` ajb!`R> . Finally we project this transformed map onto the “background map” and we renormalize thebin contents according to the map duration b` . After filling the “signal” map, the content rE of each bin iscompared to the corresponding value 5F of the background map and the Poisson probability of a backgroundfluctuation producing a bin content sO5E (when F is the expected value) is calculated.For any time duration b` the obtained probability distributions follow the expected behaviour of a uniformbackground. As an example Fig.3 and 4 show the distributions of the probabilities obtained for all bins and allmaps of duration b` =10 s and b` =300 s. No excesses with probabilities less than thuNawv= x-?y&i'oz have beenobserved for any timescale. To evaluate the sensitivity of this measurement we calculate the flux that wouldproduce an excess with probability equal to t[uNawv , using a simple model where the GRB has a spectrum dN/dE378 S.Vernetto et alspectrum slope1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3 )-2 cm-1αK (GeV-610-510-410-310-210-110110 )-2 cm-1αK (GeVCutoff energy 500 GeV 1 TeV 10 TeVGRB flux limits  Figure 5. Sensitivity to GRBs of the present ARGO-YBJdetector subset: the lines indicate the value of the normal-ization factor K of the GRB spectrum K E {$| producing anexcess of Poisson probability }~[6\ {M , as a functionof the spectral index Ł . The GRB has a duration U	V =10 sand a zenith angle qXGD . K values are given for dif-ferent cutoff energies. The points represent the values of Kand Ł of 14 EGRET GRBs.= K E &( extending from 100 GeV to a cutoff energy E uﬀ . The cutoff can be intrinsic at the source and/or dueto Extragalactic Background Light (EBL) absorption. The GRBs duration is set to 10 s and the zenith angleto 20  . Fig.5 shows the values of the normalization factor K corresponding to thuawvq -?y&('gz as a function ofthe spectral index ﬃ , for E uﬀ = 0.5, 1 and 10 TeV. For comparison in the same figure the values of K and ﬃ of14 EGRET GRBs are shown [5]. These results indicate that GRBs comparable with the most energetic eventsseen by EGRET can be observed by the present detector configuration, if their the spectrum extends with thesame slope at least up to 500 GeV. This assumption implies that only relatively close GRBs should be detected,since at larger redshifts gamma rays of higher energy are absorbed by the EBL. According to [6] the flux of500 GeV gamma rays decreases by a factor  2 (  180) if the source distance is  =0.1 (0.5), and the absorptionincreases with the gamma ray energy.5. ConclusionsThe analysis of the first data taken by an ARGO-YBJ subset of area  1900 mshows that the detector isworking properly. A preliminary sky survey searching for point sources and gamma ray transients of durationbetween 10 and 300 s shows no statistically significant excess during 1.0 10  hours of measurement in thedeclination range 	36!3 .We expect for the final detector configuration (with an area more than 3 times larger and the lead converterlayer) an improved angular resolution, a lower energy threshold and a a significant increase in the gamma raydetection sensitivity.References[1] Cao Zen et al.,”Status of the ARGO-YBJ experiment”, these proceedings.[2] G.Di Sciascio et al., Proc.28th ICRC, 3015 (2003)[3] D.E.Alexandreas et al., NIM A328, 570 (1993).[4] S.Vernetto et al., Proc 28th ICRC, 3007 (2003).[5] J.R.Catelli et al., AIP Conf.Proc.,428, 309 (1997).[6] M.H.Salamon and F.W.Stecker, ApJ 493, 547 (1998).

Sky survey with the ARGO-YBJ detector

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