The Crab is a standard calibration source for TeV gamma ray astronomy. Its energy spectrum extends beyond 10s of TeV, however, an open question is does the spectrum cutoff and at what energy. We present new results from Milagro analysis of the energy spectrum of the Crab nebula up to 100 TeV with good statistical determination. A robust algorithm was developed to estimate the energies of gamma rays on an event by event basis. This energy algorithm is based upon correlation of incident energy with core distance, zenith angle and the number of PMTs hit in the air shower and outrigger layers of Milagro telescope as determined by simulations using CORSIKA and GEANT. The variables used for energy estimation are relatively independent of the quantities used to make the gamma/hadron separation cut. The energy algorithm is used to measure the spectral slope of the background cosmic ray triggers in the full 1 to 100 TeV energy range, which gives a fit with a single spectral slope of 2.69+-0.08, showing that the algorithm works well over this energy range. The procedure is then used to obtain the energy spectrum for the Crab signal. The energy spectrum of the Milagro data is fitted using the HEGRA power index, together with an exponential cutoff at higher energies to obtain a cutoff energy of 24 +- 3 TeV.

Milagro Collaboration

Proceedings of the 30th International Cosmic Ray Conference; Rogelio Caballero, Juan Carlos D'Olivo, Gustavo Medina-Tanco, Lukas Nellen, Federico A. Sánchez, José F. Valdés-Galicia (eds.); Universidad Nacional Autónoma de México, Mexico City, Mexico, 2008; Vol. 2 (OG part 1), pages 751-754