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) x 10(-6) cm(-2) s(-1) (E > 100 MeV) was observed. The corresponding isotropic luminosity is 2.9 x 10(49) ergs s(-1). We find that PKS 1622-297 exhibits gamma-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 similar to 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 gamma-rays). This inconsistency suggests that the gamma-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 gamma-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 gamma-ray spectrum appears to evolve from hard to soft as a flare progresses.