Mostafa, M A

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29th International Cosmic Ray Conference Pune (2005) 7, 369372The Hybrid Performance of the Pierre Auger ObservatoryM.A. Mostafa´ for the Pierre Auger CollaborationObservatorio Pierre Auger, Av. San Mart·n Norte 304, Malarg¤ue, (5613) Mendoza, ArgentinaPresenter: Miguel A. Mostafa´ (mostafa@phys.unm.edu), usa-mostafa-M-abs1-he14-oralThe Pierre Auger Observatory detects ultra-high energy cosmic rays by implementing two complementary air-shower techniques. The combination of a large ground array and fluorescence detectors, known as the hybridconcept, means that a rich variety of measurements can be made on a single shower, providing much improvedinformation over what is possible with either detector alone. In this paper the hybrid reconstruction approachand its performance are described.1. IntroductionThe Pierre Auger Observatory was designed to observe, in coincidence, the shower particles at ground andthe associated fluorescence light generated in the atmosphere. This is achieved with a large array of waterCherenkov detectors coupled with air-fluorescence detectors (eyes) that overlook the surface array. It is notsimply a dual experiment. Apart from important cross-checks and measurement redundancy, the two tech-niques see air showers in complementary ways. The ground array measures the lateral structure of the showerat ground level, with some ability to separate the electromagnetic and muon components. On the other hand,the fluorescence detector records the longitudinal profile of the shower during its development through theatmosphere.A hybrid event is an air shower that is simultaneously detected by the fluorescence detector and the groundarray. The Observatory was originally designed and is currently being built with a cross–triggering capability.Data are recovered from both detectors whenever either system is triggered. If an air shower independentlytriggers both detectors the event is tagged accordingly. There are cases where the fluorescence detector, havinga lower energy threshold, promotes a sub–threshold array trigger. Surface stations are matched by timing andlocation. This is an important capability because these sub–threshold hybrid events would not have triggeredthe array otherwise.The Observatory started operation in hybrid production mode in January, 2004. Surface stations have a %duty cycle, while fluorescence eyes can only operate on clear moonless nights. Both surface and fluorescencedetectors have been running simultaneously of the time. The number of hybrid events represents thestatistics of the surface array data.2. The Hybrid PerformanceA hybrid detector has excellent capability for studying the highest energy cosmic ray air showers. Much ofits capability stems from the accurate geometrical reconstructions it achieves. Timing information from evenone surface station can much improve the geometrical reconstruction of a shower over that achieved using onlyeye pixel information. The axis of the air shower is determined by minimizing a 	 function involving datafrom all triggered elements in the eye and at ground. The reconstruction accuracy is better than the groundarray counters or the single eye could achieve independently [1, 2]. Using the timing information from theeye’s pixels together with the surface stations, a core location resolution of m is achieved. The resolutionfor the arrival direction of cosmic rays is [2]. These results for the hybrid accuracy are in good agreement370 M.A. Mostaf·a for the Pierre Auger Collaborationwith estimations using analytic arguments [3], measurements on real data using a bootstrap method [4], andprevious simulation studies [5].The reconstruction uncertainties are evaluated using events with known geometries, i.e. laser beams. TheCentral Laser Facility (CLF), described in Ref. [6], is located approximately equidistant from the first threefluorescence sites (see figure 5). Since the location of the CLF and the direction of the laser beam are known toan accuracy better than the expected angular resolution of the fluorescence detector, laser shots from the CLFcan be used to measure the accuracy of the geometrical reconstruction. Furthermore, the laser beam is splitand part of the laser light is sent through an optical fiber to a nearby ground array station. The resolution of themonocular and hybrid reconstructions are compared in figure 1 for the distance between the eye and the CLF,and in figure 2 for the angle of the axis. (distance to axis) [m]∆-2000 -1000 0 1000 2000number of laser events per 200 m (monocular)0510152025303520 m (hybrid)        Entries 168MonocularMean -271 mRMS   566 mHybridMean   27 mRMS    57 mFigure 1. Difference between the reconstructed and truedistance from the eye to the vertical laser beam using themonocular and hybrid techniques. The location of the laseris known to  m. angle [deg]∆-3 -2 -1 0 1 2 3 4 (monocular)onumber of laser events per 0.200510152025303540 (hybrid)        o0.05Entries 168MonocularoMean  0.5oRMS   1.0HybridoMean  -0.02oRMS    0.18Figure 2. Angular difference between reconstructed andtrue direction of the laser beam using the monocular andhybrid techniques. The laser beam is vertical within   .The laser light from the CLF produces simultaneous triggers in both the surface and (three) fluorescence de-tectors. The recorded event times can be used to measure and monitor the relative timing between the twodetectors. The time offset between the first fluorescence eye and the surface detector is shown in figure 3. Thistime offset has been measured to better than ns [7]. The contribution to the systematic uncertainty in thecore location due to the uncertainty in the time synchronization is m.Hybrid events can be well reconstructed with as little as one surface station. Thus, the energy threshold ofhybrid events is lower than it is for surface array only events, where at least three stations must be triggered.The energy distribution for hybrid events is shown in figure 4. Sub–threshold hybrid events have a lower energydistribution, while the angular resolution is still . Another important consequence is the additional numberof events. Approximatelyof the total hybrid events have fewer than three stations, i.e. for each event thatcan independently trigger both the surface array and one fluorescence detector there are two extra low energyevents.Hybrid Performance of the Pierre Auger Observatory 371Jan 2004 Apr Jul Oct Jan 2005 AprOffset [ns]100200300400500Figure 3. Time offset between the surface detector andLos Leones (eye 1). The variation in time is within theuncertainty of  ns.Energy [eV]1710 1810 1910 2010Number of hybrid events per log(E)/10110210All hybrid eventsSub-threshold evts.Figure 4. Energy histogram for hybrid events. The shadedhistogram represents the energy distribution for the subset of hybrid events with fewer than three surface stations.These events are referred to as subthreshold events.East  [km]0 10 20 30 40 50 60North  [km]0102030405060Eye 1Los LeonesEye 4CoihuecoEye 2Los MoradosJune 14, 2005CLFFigure 5. Status of the Pierre Auger Observatory on June14, 2005. Each gray full circle represents a surface sta-tion, each color dot is the core location of a reconstructedair shower (where the color indicates which eye saw thatevent), and each uorescence site is shown as a coloredsemi-circle. The Central Laser Facility is shown as a greenfull star.The status of the Observatory on June 14, 2005 is summarized in figure 5. There were 740 fully operationaltanks at that time. The first two fluorescence sites (eyes andin the figure) were fully operational, i.e.running six telescopes each, in June, 2004. The third site (eye  ) started operation on March 18, 2005. Thepresent average rate is hybrid events per night per eye, for a total of 16090 events up to June 14, 2005. Atthis rate,hybrid events per month are expected when the Observatory is completed.3. Hybrid MeasurementsDue to the much improved angular accuracy, the hybrid data sample is ideal for anisotropy studies and, inparticular, for point source searches. Results on a search for a point–like source in the direction of the galacticcenter using these hybrid events are presented at this conference [8].Many ground parameters, like the shower front curvature and thickness, have always been difficult to measureexperimentally, and were usually determined from Monte Carlo simulations. The hybrid sample provides aunique opportunity in this respect. As mentioned, the geometrical reconstruction can be done using only oneground station, thus all the remaining detectors can be used to measure the shower characteristics.The combination of the air fluorescence measurements and particle detections on the ground provides an en-ergy measurement almost independent of air shower simulations. The fluorescence measurements determinethe longitudinal development of the shower, whose integral is proportional to the total energy of the electro-magnetic particle cascade. At the same time, the particle density at any given distance from the core can be372 M.A. Mostaf·a for the Pierre Auger Collaborationevaluated with the ground array. The conversion from particle density to the energy of the shower is wherethe fluorescence measurements become important. Hybrid events that can be independently reconstructed withboth techniques are used to establish an empirical rule for the energy conversion. The procedure to determinethe energy of each event is explained in more detail in Ref. [9]. It is important to note that both techniques havedifferent systematics, and results are preliminary at this stage while the Observatory is under construction. Thepossibility of studying the same set of air showers with two independent methods is valuable in understand-ing the strengths and limitations of each technique. The hybrid analysis benefits from the calorimetry of thefluorescence technique and the uniformity of the surface detector aperture.4. ConclusionsWe have tested the performance of Pierre Auger Observatory in its hybrid configuration. Operation started inJanuary, 2004 and over 16000 hybrid events have been successfully reconstructed up to now. It is importantto note that the Observatory is under construction and that results are preliminary. It is already clear that thecombination of fluorescence and ground array measurements provides reconstruction of the geometry of theshower with much greater accuracy than is achieved with either detector system on its own. Unprecedentedcore location and direction precision leads to excellent energy and shower development measurements. SeveralAuger measurements being presented at this conference profit from the hybrid capability of the Observatory,including the angular resolution [2], energy spectrum [9], and photon limits [10].References[1] Pierre Auger Collaboration, “Performance of the Fluorescence Detectors of the Pierre Auger Observa-tory,” these proceedings, aus-bellido-J-abs1-he14-oral.[2] Pierre Auger Collaboration, “Angular Resolution of the Pierre Auger Observatory,” these proceedings,bra-bonifazi-C-abs1-he14-oral.[3] P. Sommers, “Capabilities of a giant hybrid air shower detector,” Astroparticle Physics 3 (1995) 349.[4] B. Fick for the Pierre Auger Collaboration, “Hybrid Performance of the Pierre Auger Observatory andReconstruction of Hybrid Events,” astro-ph/0308512, Proceedings of the 28th International Cosmic RayConferences, Tsukuba, Japan, (2003) 449.[5] B.R. Dawson et al., “Simulations of a Giant Hybrid Air Shower Detector,” Astroparticle Physics 5 (1996)239.[6] F. Arqueros et al. for the Pierre Auger Collaboration, “The Central Laser Facility at the Pierre AugerObservatory,” these proceedings, usa-malek-M-abs1-he15-poster.[7] P. Allison et al. for the Pierre Auger Collaboration, “Time Calibration and Synchronization of Surfaceand Fluorescence Detectors of the Pierre Auger Observatory,” these proceedings, usa-covault-C-abs2-he15-poster.[8] Pierre Auger Collaboration, “Anisotropy Studies Around the Galactic Center at EeV Energies with Augerdata,” these proceedings, fra-letessier-selvon-A-abs1-he14-oral.[9] Pierre Auger Collaboration, “First Estimate of the Primary Cosmic Ray Energy Spectrum above 3 EeVfrom the Pierre Auger Observatory,” these proceedings, usa-sommers-P-he14-oral.[10] Pierre Auger Collaboration, “Upper limit on the primary photon fraction from the Pierre Auger Observa-tory,” these proceedings, ger-risse-M-abs2-he14-oral.

The Hybrid Performance of the Pierre Auger Observatory

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