A rapid X-ray flare in the radio-loud narrow-line quasar PKS 0558-504

We report the detection of a very short timescale flare in the radio-loud n arrow-line quasar PKS 0558 - 504 that was observed using ASCA. The X-ray co unt rates increased and decreased by a factor of 2 in 33 minutes and possib ly by 40% in as short a time period as 2 minutes during this flare, confirm ing with imaging detectors that such a flare event does occur in this objec t. The implied largest rate of change in luminosity in 0.8-10 keV alone, dL /dt similar or equal to (1.8 +/- 0.4) x 10(42) ergs s(-2), is several times higher than the limit set for the isotropic emitting plasma around a Kerr black hole. Emission from either a relativistic boosting jet or a magnetic heated corona may explain such high radiative efficiency. A magnetic field with a strength of at least a few times 10(4) C is required in the latter c ase. The spectrum during this flare is significantly harder than an average spectrum. Three radio-loud narrow-line active galactic nuclei (AGNs) posse ss considerably smaller black holes than the radio-loud AGNs in bright-quas ar survey, indicating that they are still in a rapid-growth phase of the bl ack hole by accretion.