We report the detection of a very short time scale flare in the radio-loud narrow line quasars PKS 0558-504 by using ASCA .
The X-ray count rates increased and decreased by a factor of two in 33 minutes , and possibly by 40 % in as short as two minutes during this flare , confirming that such flare event does occur in this object with imaging detectors .
The implied largest rate of change in luminosity in 0.8-10 keV alone , dL / dt \simeq ( 1.8 \pm 0.4 ) 10 ^ { 42 } ergs s ^ { -2 } , is several times higher than the limit sets for the isotropic emitting plasma around a Kerr black hole .
Either emission from a relativistic boosting jet or a magnetic heated corona may explain such high radiative efficiency .
Magnetic field with a strength of at least a few 10 ^ { 4 } Gauss is required in the latter case .
The spectrum during this flare is significant harder than the average one .
Three radio loud narrow line active galactic nuclei ( AGN ) possess considerably smaller black holes than in radio loud AGN in bright quasar survey , indicating they are still in a phase of the rapid growth of the black hole by accretion .