The exposure of the hybrid detector of the Pierre Auger Observatory

Abstract

a b s t r a c tThe Pierre Auger Observatory is a detector for ultra-high energy cosmic rays. It consists of a surface array to measure secondary particles at ground level and a fluorescence detector to measure the development of air showers in the atmosphere above the array. The ''hybrid" detection mode combines the information from the two subsystems. We describe the determination of the hybrid exposure for events observed by the fluorescence telescopes in coincidence with at least one water-Cherenkov detector of the surface array. A detailed knowledge of the time dependence of the detection operations is crucial for an accurate evaluation of the exposure. We discuss the relevance of monitoring data collected during operations, such ⇑

The Lateral Trigger Probability function for the Ultra-High Energy Cosmic Ray showers detected by the Pierre Auger Observatory

“…The agreement is good over the entire energy range. This feature actually starts at very low energies, even below the range of full efficiency for the hybrid detection [3]. In this case, whereas in data only events with at least one SD ToT station are selected, in simulation also the events that did not trigger at all are taken into account in the calculation of the probability.…”

“…Two years of hybrid data collected between June 2006 and May 2008 were used for this study. The events are selected as described in [3] and this ensures an angular resolution of about 0.6 • and a core position determination better than 70 m. Further requirements on the goodness of the reconstructed longitudinal profile provide an energy resolution of about 10% above 10 18 eV and less than 15% at lower energies [3]. The LTP measured from data is shown in Fig.…”

“…Since FD has a lower energy threshold, hybrid events are also detected below the minimum energy for an independent SD trigger. For zenith angles below 60 • , the hybrid detector reaches nearly full efficiency at 10 18 eV [3].…”

“…This task relies on an accurate determination of the detector exposure for SD-only [4] and hybrid [3] operation modes. The hybrid exposure is calculated using the simultaneous simulation of FD and SD response.…”

“…The fluorescence detector (FD), consisting of 24 telescopes at four sites, overlooks the surface array and observes the longitudinal profile of air showers by collecting the fluorescence light emitted along the path through the atmosphere [2]. Unlike the surface detector array with its nearly 100% duty cycle, the FD can only operate on clear and moonless nights giving an overall duty cycle of about 13% [3]. As a consequence, only a fraction of showers are observed by both detectors.…”