Abstract
Abstract The Crab Nebula is the brightest TeV gamma-ray source in the sky and has been used for the past 25 years as a reference source in TeV astronomy, for calibration and verification of new TeV instruments. The High Altitude Water Cherenkov Observatory (HAWC), completed in early 2015, has been used to observe the Crab Nebula at high significance across nearly the full spectrum of energies to which HAWC is sensitive. HAWC is unique for its wide field of view, nearly 2 sr at any instant, and its high-energy reach, up to 100 TeV. HAWC’s sensitivity improves with the gamma-ray energy. Above ∼1 TeV the sensitivity is driven by the best background rejection and angular resolution ever achieved for a wide-field ground array. We present a time-integrated analysis of the Crab using 507 live days of HAWC data from 2014 November to 2016 June. The spectrum of the Crab is fit to a function of the form . The data is well fitted with values of α = 2.63 ± 0.03, β = 0.15 ± 0.03, and when E 0 is fixed at 7 TeV and the fit applies between 1 and 37 TeV. Study of the systematic errors in this HAWC measurement is discussed and estimated to be ±50% in the photon flux between 1 and 37 TeV. Confirmation of the Crab flux serves to establish the HAWC instrument’s sensitivity for surveys of the sky. The HAWC all-sky survey will be the deepest survey of the northern sky ever conducted in the multi-TeV band.
Highlights
The Crab Pulsar Wind Nebula occupies a place of special distinction in the history of high-energy astrophysics
∼1 TeV the sensitivity is driven by the best background rejection and angular resolution ever achieved for a wide-field ground array
A 10 TeV photon will hit nearly every sensor and the B variable has no dynamic range above this energy. This limit is not intrinsic to High Altitude Water Cherenkov Observatory (HAWC) and variables that utilize the light level seen in photomultiplier tubes (PMTs) on the ground, similar to what was used in the original sensitivity study (Abeysekara et al 2013), have dynamic range above 100 TeV
Summary
The Crab Pulsar Wind Nebula (the Crab Nebula or the Crab) occupies a place of special distinction in the history of high-energy astrophysics It was the first high-confidence TeV detection in 1989 using the Whipple telescope (Weekes et al 1989) and is the brightest steady source in the Northern sky above 1 TeV. The Crab Nebula has been adopted as the reference source in TeV astronomy and is a reliable beam of high-energy photons to use for calibrating and understanding new TeV gamma-ray instruments. The High Altitude Water Cherenkov (HAWC) observatory is a new instrument sensitive to multi-TeV hadron and gamma-ray air showers, operating at latitude of +19◦N at an altitude of 4,100 meters in the Sierra Negra, Mexico. A high-energy photon impinging on the atmosphere above HAWC initiates an extensive electromagnetic air shower.
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