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 \left ( \frac { dN } { dE } = \phi _ { 0 } \left ( E / \textrm { 7 TeV } \right ) ^ { - \alpha - \beta \ln% \left ( E / \textrm { 7 TeV } \right ) } \right ) 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 meters from the shower axis , the best-fit values are \phi _ { o } = ( 2.35 \pm 0.04 ^ { +0.20 } _ { -0.21 } ) \times 10 ^ { -13 } ( TeV cm ^ { 2 } s ) ^ { -1 } , \alpha =2.79 \pm 0.02 ^ { +0.01 } _ { -0.03 } , and \beta =0.10 \pm 0.01 ^ { +0.01 } _ { -0.03 } .
For the second estimator , a neural network which uses the charge distribution in annuli around the core and other variables , these values are \phi _ { o } = ( 2.31 \pm 0.02 ^ { +0.32 } _ { -0.17 } ) \times 10 ^ { -13 } ( TeV cm ^ { 2 } s ) ^ { -1 } , \alpha =2.73 \pm 0.02 ^ { +0.03 } _ { -0.02 } , and \beta =0.06 \pm 0.01 \pm 0.02 .
The first set of uncertainties are statistical ; the second set are systematic .
Both methods yield compatible results .
These measurements are the highest-energy observation of a gamma-ray source to date .