ROSAT High-Resolution Imager monitoring of extreme X-ray variability in the narrow-line quasar PHL 1092

We report results from an 18-d ROSAT High-Resolution Imager (HRI) monitoring campaign on the ultrasoft narrow-line Seyfert 1 (NLS1) class quasar PHL 1092. This luminous, radio-quiet quasar showed strong X-ray variability in a short ROSAT Position-Sensitive Proportional Counter observation, and ROSAT HRI monitoring of the similar object IRAS 13224-3809 revealed extreme variability on intermediate rime-scales. We wanted to determine whether remarkable X-ray variability persistently occurs in PHL 1092, and we also wanted to search for outstanding variability events that constrain emission processes. Given the large luminosity of PHL 1092 (similar to 5 x 10(45) erg s(-1) in the HRI band), we detect extremely rapid and large-amplitude X-ray variability throughout our monitoring. The maximum observed variability amplitude is a factor of approximate to 14, and in the most rapid variability event the HRI count rate increases by a facto; of approximate to 3.8 in a rest-frame time interval of < 3580 s, The most rapid event has a rate change of luminosity of > 1.3 x 10(42) erg s(-2), making it the most extreme such event we are aware of from a radio-quiet quasar. Standard 'radiative efficiency limit' arguments imply a radiative efficiency larger than can be achieved by accretion on to a Kerr black hole rotating at the maximum plausible rate, although we point out that such arguments depend upon the geometry of initial radiation release. Relativistic motions of the X-ray source are probably causing the radiative efficiency limit to break down; such relativistic motions;have also been inferred in the similar NLS1-class quasar PKS 0558-503.