An Intense Gamma‐Ray Flare of PKS 1622−297

Abstract

We report the observation by the Compton Gamma Ray Observatory of a spectacular flare of radio source PKS1622−297. A peak flux of (17 ± 3) × 10 −6 cm −2 s −1 (E> 100 MeV) was observed. The corresponding isotropic luminosity is 2.9×10 49 erg/s. We find that PKS 1622−297 exhibits γ-ray intra-day variability. A flux increase by a factor of at least 3.6 was observed to occur in less than 7.1 hours (with 99 % confidence). Assuming an exponential rise, the corresponding doubling time is less than 3.8 hours. A significant flux decrease by a factor of ∼2 in 9.7 hours was also observed. Without beaming, the rapid flux change and large isotropic luminosity are inconsistent with the Elliot-Shapiro condition (assuming that gas accretion is the immediate source of power for the γ-rays). This inconsistency suggests that the γ-ray emission is beamed. A minimum Doppler factor of 8.1 is implied by the observed lack of pair-production opacity (assuming x-rays are emitted co-spatially with the γ-rays). Simultaneous observation by EGRET and OSSE finds a spectrum adequately fit by a power law with photon index of −1.9. Although the significance is not sufficient to establish this beyond doubt, the high-energy γ-ray spectrum appears to evolve from hard to soft as a flare progresses. quasars:general — quasars:individual:PKS 1622−297 — gamma rays: observations