Very High Energy Photons Research Articles

Intrinsic Constraints on Very High Energy Emission in Gamma‐Ray–loud Blazars

Photons with very high energy (VHE) up to TeV emitted from active galactic nuclei (AGNs) provide some invaluable information on the origin of gamma-ray emission. Although 66 blazars have been detected by EGRET, only three low-redshift X-ray-selected BL Lac objects (Mrk 421, Mrk 501, and 1ES 2344+514) are conclusive TeV emitters (PKS 2155-304 is a potential TeV emitter), since VHE photons may be absorbed by cosmological background infrared photons (external absorption). Based on the mirror effect of clouds in the broad-line region, we argue that there is an intrinsic mechanism for the deficiency of TeV emission in blazars. Employing the observable quantities, we derive the pair production optical depth tau(gamma gamma)(epsilon(obs)) due to the interaction of VHE photons with the reflected synchrotron photons by electron Thomson scattering in the broad-line region. This sets stronger constraints on very high energy emission, and provides a sensitive upper limit on the Doppler factor of the relativistic bulk motion. It has been suggested that the intrinsic absorption be distinguished from the external by the observation on variation of the multiwavelength continuum.

Could very high energy gamma rays from the Crab Nebula result from p-p collision?

It is generally believed that the very high energy (VHE) gamma rays emitted from the Crab Nebula cannot result from the decay of neutral pions which are produced by the collision between relativistic protons, due to the small hadronic cross section and low column density of the nebula. However, the magnetic field of the Crab Nebula could trap a large amount of relativistic protons from the pulsar with a total energy of up to 1047 erg, because this amount of energy is difficult to release for the same reason. Nevertheless, a small amount of protons will be converted to VHE photons through occasional hadronic collisions which result in steady VHE gamma ray emission with an average power of approximately 4*1034 erg s-1. The authors calculate the proton energy spectrum by using the pulsar evolution model proposed by Ruderman and Cheng (1988).