First Detection of a Pulsar above 100 GeV

TL;DR

This paper reports the first detection of pulsed gamma-ray emission from the Crab Pulsar above 100 GeV, challenging existing pulsar emission models and suggesting emission occurs beyond 10 stellar radii.

Contribution

It provides the first observational evidence of pulsed gamma rays above 100 GeV from a pulsar, with spectral analysis indicating a power-law spectrum and ruling out curvature radiation.

Findings

Detection of pulsed emission above 100 GeV from the Crab Pulsar.

Spectrum described by a broken power law, not an exponential cutoff.

Emission region located beyond 10 stellar radii from the neutron star.

Abstract

We present the detection of pulsed gamma-ray emission from the Crab Pulsar above 100 GeV with the VERITAS array of atmospheric Cherenkov telescopes. Gamma-ray emission at theses energies is not expected in present pulsar models. We find that the photon spectrum of pulsed emission between 100 MeV and 400 GeV can be described by a broken power law, and that it is statistically preferred over a power law with an exponential cut-off. In the VERITAS energy range the spectrum can be described with a simple power law with a spectral index of -3.8 and a flux normalization at 150 GeV that is equivalent to 1 % of the Crab Nebula gamma-ray flux. The detection of pulsed emission above 100 GeV and the absence of an exponential cutoff rules out curvature radiation as the primary gamma-ray-producing mechanism. The pulse profile exhibits the characteristic two pulses of the Crab Pulsar at phases 0.0 and 0.4, albeit 2-3 times narrower than below 10 GeV. The narrowing can be interpreted as a tapered particle acceleration region in the magnetosphere. Our findings require that the emission region of the observed gamma rays be beyond 10 stellar radii from the neutron star.