First HAWC Spectra of Galactic Gamma-ray Sources Above 100 TeV and the Implications for Cosmic-ray Acceleration

Abstract

We present the first catalogs of the highest-energy (above 56 TeV and 100 TeV) gamma-ray sources seen by the High Altitude Water Cherenkov (HAWC) Observatory. The wide field-ofview of HAWC naturally lends itself to unbiased all-sky surveys and newly developed event-byevent gamma-ray energy reconstruction algorithms have allowed unprecedented energy resolution. The sources presented here are the highest-energy sources ever detected. All are coincident with known lower-energy gamma-ray sources within our Galaxy. These objects may have implications for the sources of Galactic cosmic rays; since Galactic CRs have been observed up to PeV energies, sources accelerating particles to these energies must exist. These sources, called "PeVatrons", would have corresponding hard gamma-ray spectra that extend to high energies without any spectral break or cutoff. We will present measurements of the spectra of these highest-energy gamma-ray sources and discuss if any of them can be identified as PeVatron candidates.

Summary of the 36th ICRC Gamma-ray Indirect sessions

“…Three additional sources have been detected above 100 TeV, however proving if they are indeed PeVatrons will require careful modelling of the emission. It is interesting to note that those sources are close to radio pulsars, pointing to PWN or pulsar halo origin, and hence leptonic rather than hadronic origin of the emission [44].…”

“…[41,42] makes it even more interesting to investigate emission at those energies. All three instruments MAGIC [43], HAWC [44] and Tibet AS+MD [45] array reach (or go beyond) energies of 100 TeV without any visible cutoff (See Fig. 3).…”

“…Individual photons with estimated energies of a few hundred TeV were reported, however the energy resolution as well as possible misclassified events should be taken into account when interpreting those. HAWC performed a search of other sources emitting above 100 TeV [44]. Three additional sources have been detected above 100 TeV, however proving if they are indeed PeVatrons will require careful modelling of the emission.…”

Testing the Limits of Particle Acceleration in Cygnus OB2 with HAWC

“…Currently HAWC utilises two different methods of estimating gamma-ray energy. One is called the "ground parameter" which uses charge density at 40 m from the shower axis [17]. Another method is called the "neural network" which relies on an artificial neural network and various parameters from the event reconstrunction process [18].…”

The Spectrum of the Crab Nebula and Highest Energy Photons Measured by HAWC

“…We consider, besides the Crab, six other sources [14] which have evidence of emission at high energy: although the Crab is the brightest VHE source, other sources with a harder spectrum might be sources of higher-energy photons. The analysis needs a spectrum model for each source.…”