Cosmology with Ground-Based VHE Gamma-Ray Detectors
Session 10 -- Gamma Ray Sources
Display presentation, Monday, 30, 1994, 9:20-6:30

## [10.02] Cosmology with Ground-Based VHE Gamma-Ray Detectors

P.S. Coppi (University of Chicago), F.A. Aharonian, H. V\"olk (Max-Planck-Institut f\"ur Kernphysik)

Active Galactic Nuclei (AGN) and Superclusters of Galaxies, considered to be possible sources for the observed flux of extremely high energy cosmic rays ($E \geq 10^{19} \, \rm eV$), may well have $\gamma$-ray emission extending into the VHE (very high energy, $E_\gamma>100$ GeV) domain. Because VHE $\gamma$-rays are absorbed by pair production on the intergalactic background radiation fields, much of this emission may not be directly visible. The electromagnetic cascade initiated by the absorbed VHE $\gamma$-rays, however, is observable. Since, the velocities of $e^{+}e^{-}$ pairs produced in the cascade are most probably isotropized by an ambient random magnetic field, extended `halos' ($R > 1 \, \rm Mpc$) of pairs will be formed around AGN with VHE emission. The cascade radiation from these pair halos is emitted almost isotropically and should be observable at energies below a few TeV. The $\gamma$-ray flux from a halo depends mainly on the total luminosity and duty cycle of the source at energies $\geq 10$ TeV, but not on the source geometry, in particular the direction and beaming/opening angle of a $\gamma$-ray emmiting jet. (The halo emission is not beamed.) The halo radiation can be distinguished by its characteristic variation in spectrum and intensity with angular distance from the central source. This depends on the distance to the source and the level of the $IR/O$ background of the local enviroment. Thus, the investigation of the angular and spectral distribution of halo radiation could provide almost model-independent and unambiguous cosmological information about the universal $IR/O$ background, the Hubble constant, and the VHE power of AGN. Next-generation ground-based systems (arrays) of low threshold ($E \geq 100 \, \rm GeV$) imaging Cherenkov telescopes will be able to probe the VHE apparent luminosities of extragalactic sources down to $L \approx 10^{43}(d/1000 \, \rm Mpc)^{2} \, \rm erg/s$, which is two orders of magnitude lower than the minimum luminosity detectable by EGRET at GeV energies. The excellent angular resolution expected from these systems ($\approx 0.1 \deg$) will allow us to image pair halos of even cosmologically distant ($z \geq 1$) sources.