Neil Gehrels's scientific contributions

In the paper "Two Classes of Gamma-Ray-emitting Active Galactic Nuclei" by Charles D. Dermer and Neil Gehrels (ApJ, 447, 103 [1995)), the following corrections and clarifications should be made. (l) In Table 3, the values of apparent transverse superluminal speeds given in the column labeled "hβ_app_ + h {DELTA}β_app_ should be multiplied by (4/3)^2^. (2) In equation (3), a brace is missing following the δ symbol. (3) In equation (6), the term -1 should be deleted in the integration over x', and the final term should be written n_ph_(η, μ; x'). (4) The values of F_-6_ in Table 4 are average flux values during the flaring episode; the correct value of τgammaγ_(E_100_ = 0.01) for 3C 279 is 7.3 x 10^-3^. (5) The correct values of M_8,min_^EGRET_ in Table 5 for 3C 273, 1633+382, and 0528 + 134 are 0.0014, 0.60, and 0.60, respectively. (6) The term s refers both to photon spectral index and the invariant 2ηη_1_(1 - μ).

We interpret recent gamma-ray observations of active galactic nuclei (AGNs) made with the Whipple Observatory, Granat and especially the Compton Gamma Ray Observatory. The gamma-ray data show that there are two distinct classes of AGNs defined by their redshift and luminosity distributions and high-energy spectral properties. Sources in the first class, which are generally associated with AGNs classified in other wavelength ranges as Seyferts, have redshifts z ≲ 0.06 and 50-150 keV luminosities in the range 1041-1044 ergs s-1. These sources display spectral softenings at ˜100 keV energies, with no measured emission at photon energies E > several MeV. This class includes radio-quiet AGNs in addition to radio galaxies apparently viewed at large angles with respect to the radio jet axis. The redshifts of objects in the second class, which are associated with AGNs classified as blazars, are as large as z ≅ 2.3, and the range of 100 MeV-5 GeV luminosities, assuming isotropic emission, extends to 1049 ergs s-1. The ≈20 MeV-30 GeV gamma-ray luminosity often dominates the bolometric luminosity in objects of this class. These sources probably represent AGNs that are observed nearly along the axis of a radio jet. Some AGNs show evidence from the high-energy data for transitional behavior between the two classes. We consider whether the qualitatively different properties of the two gamma-ray classes provide evidence for quasi-isotropic emission from the Seyferts and beamed emission from the blazars. Comparison of the observed redshift and luminosity distributions with model distributions derived from a treatment of the cosmological statistics of isotropic and beamed sources gives, however, inconclusive results. We treat gamma- ray transparency arguments for beaming, avoiding earlier unproven assumptions that X-rays and E > 100 MeV gamma rays originate from the same site. The pair-production optical depth of E > 100 MeV gamma rays interacting with other gamma rays is much less than 1 and does not require beaming, but data from OSSE give evidence for beaming in a few blazars. We generalize to the gamma-ray regime the Elliot-Shapiro relation, which is based on the assumption that AGN radiation is isotropically emitted and that the luminosity is generated by Eddington-limited accretion. Available gamma-ray data do not yet demonstrate a strong conflict with this limit. The generalization of the Eddington-luminosity limit to the Klein-Nishina limit suggests, however, a new type of object that can accrete at luminosities much greater than 1046 M8 ergs s-1 by radiating photons at gamma-ray energies. Here M8 is the black hole mass in units of 108 Msun. Beaming arguments from gamma-ray observations require more observations of blazars but superluminal observations probably still provide the most compelling evidence for bulk relativistic motion in blazars.