Detection of Pulsed Gamma Rays Above 100 GeV from the Crab Pulsar

Abstract

We report the detection of pulsed gamma rays from the Crab pulsar at energies above 100 giga-electron volts (GeV) with the Very Energetic Radiation Imaging Telescope Array System (VERITAS) array of atmospheric Cherenkov telescopes. The detection cannot be explained on the basis of current pulsar models. The photon spectrum of pulsed emission between 100 mega-electron volts and 400 GeV is described by a broken power law that is statistically preferred over a power law with an exponential cutoff. It is unlikely that the observation can be explained by invoking curvature radiation as the origin of the observed gamma rays above 100 GeV. Our findings require that these gamma rays be produced more than 10 stellar radii from the neutron star.

Phase-resolved energy spectra of the Crab pulsar in the range of 50–400 GeV measured with the MAGIC telescopes

“…It is one of the few pulsars that have been detected in almost all energies, ranging from radio (e.g., Lyne et al 1993) to VHE gamma rays. In the highest-energy regime, it was detected up to a few tens of GeV by Fermi-LAT (Abdo et al 2010a), between approximately 25−100 GeV by MAGIC Saito 2010;Aleksić et al 2011) and above 100 GeV by VERITAS (Aliu et al 2011). The light curves and the spectra obtained by these observations suggest that gamma-ray pulsars have high-altitude emission zones that avoid a super-exponential spectral cutoff, which would be caused by magnetic pair production.…”

“…In this scenario, the cutoff energy corresponds to the highest characteristic curvature-radiation energy of the particles accelerated in the magnetosphere (e.g., Romani 1996). However, the spectrum of the Crab pulsar strongly disfavors an exponential cutoff (Aleksić et al 2011;Aliu et al 2011), making this pulsar a counterexample of the general property. Thus, to develop pulsar emission theories beyond the widely accepted curvature-radiation models, it is essential to examine the detailed phase-resolved spectrum of this youngest pulsar in the HE to VHE regimes.…”

“…The difference between them is mainly that the wide intervals lead to a higher noise contribution but are free of any possible selection bias, whereas the narrow intervals have much lower noise, but are affected by a minor selection bias. The VERITAS results shown in Aliu et al (2011) were calculated using P1 V = [0.987−0.009] and P2 V = [0.375−0.421], which is still a bit narrower than our definitions.…”

“…Consequently, the excess we found is much more concentrated than the wide peak ranges defined in Fierro et al (1998) (P1 E = [0.94−0.04] and P2 E = [0.32−0.43], where E stands for EGRET, in contrast to the MAGIC and VERITAS definitions below). Because with too-large phase intervals one integrates an unnecessarily large number of noise events, we decided to investigate the signal both in the EGRET intervals and in narrower, a posteriori defined phase intervals, using the Gaussian peak positions ±2 σ, as was done in Aliu et al (2011) Li & Ma (1983, Eq. (17), see also Table 1).…”

H.E.S.S.-II - Gamma ray astronomy from 20 GeV to hundreds of TeV’s

“…Later detection of pulsed emission above 100 GeV [10,11], and up to at least 1 TeV [12] implies that the emission takes place in the vicinity of the light cylinder, and triggered an intense theoretical and experimental activity.…”