A RAPID ENERGETIC X-RAY FLARE IN THE QUASAR PKS0558-504

QUASARS normally exhibit only small fluctuations in brightness, but occasional instances of substantial variability have been used to constrain the size and geometry of the emitting region. Here we report a recent observation from the Ginga satellite 1 of the quasar PKS0558-504, during which the X-ray flux increased by 67% in the space of only 3 minutes. There was no significant change in the spectrum. Comprehensive analysis of the data strongly indicates that this was a genuine X-ray flare originating in the quasar. The implied rate of change in luminosity in the 2-10 keV range, assuming a Hubble constant of 70 km s-1 Mpc-1 and a cosmological deceleration parameter q0 = 0.5, is 3.2 x 10(42) erg s-2, the highest value measured for a quasar. When photon scattering is considered, this is approximately 16 times greater than could be produced, with a 3-minute rise time, in an isotropically emitting plasma. We argue that the apparent luminosity must be enhanced by relativistic beaming. This is the first indication of beaming in an 'ordinary' unpolarized quasar.