A Rapid X-ray Flare in the Radio-loud Narrow Line Quasar PKS 0558-504

T.G. Wang$^1$, M. Matsuoka$^2$, H. Kubo$^3$, T. Mihara$^4$, 
\& H. Negoro$^4$

1. Center for Astrophysics, University of Science \& Technology of China,
Hefei, Anhui 230026, China\newline
2. NASDA\newline
3. Tokyo Institute of Technology\newline
4. RIKEN

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 the flare, confirming that such
flare event does not occur in this object by an imaging detector. 
The implied largest rate of change in luminosity in 0.8--10 keV alone,
$dL/dt \simeq (1.8 \pm 0.4)10^{42}$ erg/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 load narrow line quasars possess considerably
smaller than black holes than in radio loud queasars, indicating they
are still in a phase of the rapid growth of the black hole by accretion.