An Intense Gamma-Ray Flare of PKS 1622–297

We report the observation by the Compton Gamma Ray Observatory of a spectacular flare of radio source PKS 1622-297. A peak flux of (17 ± 3) × 10-6 cm-2 s-1 (E > 100 MeV) was observed. The corresponding isotropic luminosity is 2.9 × 1049 ergs s-1. We find that PKS 1622-297 exhibits γ-ray intraday variability. A flux increase by a factor of at least 3.6 was observed to occur in less than 7.1 hr (with 99% confidence). Assuming an exponential rise, the corresponding doubling time is less than 3.8 hr. A significant flux decrease by a factor of ~2 in 9.7 hr 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 cospatially with the γ-rays). Simultaneous observation by EGRET and OSSE finds a spectrum adequately fitted 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.