OBSERVATION AND SPECTRAL MEASUREMENTS OF THE CRAB NEBULA WITH MILAGRO

The Crab Nebula was detected with the Milagro experiment at a statistical significance of 17 standard deviations over the lifetime of the experiment. The experiment was sensitive to approximately 100 GeV–100 TeV gamma-ray air showers by observing the particle footprint reaching the ground. The fraction of detectors recording signals from photons at the ground is a suitable proxy for the energy of the primary particle and has been used to measure the photon energy spectrum of the Crab Nebula between ∼1 and ∼100 TeV. The TeV emission is believed to be caused by inverse-Compton upscattering of ambient photons by an energetic electron population. The location of a TeV steepening or cutoff in the energy spectrum reveals important details about the underlying electron population. We describe the experiment and the technique for distinguishing gamma-ray events from the much more-abundant hadronic events. We describe the calculation of the significance of the excess from the Crab and how the energy spectrum is fitted. The differential photon energy spectrum, including the statistical errors from the fit, obtained using a simple power-law hypothesis for data between 2005 September and 2008 March is (6.5 ± 0.4) × 10−14(E/10 TeV)−3.1±0.1(cm2 s TeV )−1 between ∼1 TeV and ∼100 TeV. Allowing for a possible exponential cutoff, the photon energy spectrum is fitted as (2.5+0.7− 0.4) × 10−12(E/3 TeV)−2.5 ± 0.4exp (− E/32+39− 18 TeV) (cm2 s TeV)−1. The results are subject to an ∼30% systematic uncertainty in the overall flux and an ∼0.1 systematic uncertainty in the power-law indices quoted. Uncertainty in the overall energy scale has been absorbed into these errors. Fixing the spectral index to values that have been measured below 1 TeV by IACT experiments (2.4–2.6), the fit to the Milagro data suggests that Crab exhibits a spectral steepening or cutoff between about 20–40 TeV.