...

Present address: GAM, CC 085 - Bât. 11, Univ. de Montpellier II, Place Eugène Bataillon, 34095 Montpellier Cedex 5, France.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

...

Present address: Institut des Sciences Nucléaires, 53 avenue des Martyrs, 38026 Grenoble Cedex, France.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

...

Present address: IGPP, University of California, Riverside, CA 92521, USA.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

... Zenith

The threshold is defined here following the usual convention for atmospheric Cherenkov detectors, i.e. as the energy at which the differential $\gamma$-ray rate is maximum at the trigger level for a Crab-like spectrum. As shown in Fig. 2, it is somewhat higher after the event selection applied at the analysis level.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.