Measurement of the Crab Nebula Spectrum Past 100 TeV with HAWC

We present TeV gamma-ray observations of the Crab Nebula, the standard reference source in ground-based gamma-ray astronomy, using data from the High Altitude Water Cherenkov (HAWC) Gamma-Ray Observatory. In this analysis we use two independent energy estimation methods that utilize extensive air shower variables such as the core position, shower angle, and shower lateral energy distribution. In contrast, the previously published HAWC energy spectrum roughly estimated the shower energy with only the number of photomultipliers triggered. This new methodology yields a much-improved energy resolution over the previous analysis and extends HAWC's ability to accurately measure gamma-ray energies well beyond 100 TeV. The energy spectrum of the Crab Nebula is well fit to a log-parabola shape (dN/dE = φ0 (E/7 TeV)-α-β In (E/7 TeV) with emission up to at least 100 TeV. For the first estimator, a ground parameter that utilizes fits to the lateral distribution function to measure the charge density 40 m from the shower axis, the best-fit values are φ0 = (2.35 ± 0.04-0.21+0.20) × 10-13 (TeV cm2 s)-1, α = 2.79 ± 0.02-0.03+0.01, and β = 0.10 ± 0.01-0.03+0.01. For the second estimator, a neural network that uses the charge distribution in annuli around the core and other variables, these values are φ0 = (2.31 ± 0.02-0.17+0.32) × 10-13(TeV cm2 s)-1, α = 2.73 ± 0.02-0.02+0.03, and β = 0.06 ± 0.01 ± 0.02. The first set of uncertainties is statistical; the second set is systematic. Both methods yield compatible results. These measurements are the highest-energy observation of a gamma-ray source to date.