Intranight polarization variability in radio-loud and radio-quiet AGN
  (Abriged) Intranight polarization variability in AGN has not been studied
extensively so far. Studying the variability in polarization makes it possibly
to distinguish between different emission mechanisms. Thus it can help
answering the question if intranight variability in radio-loud and radio-quiet
AGN is of the same or of fundamentally different origin. In this paper we
investigate intranight polarization variability in AGN. Our sample consists of
28 AGN at low to moderate redshifts (0.048 < z < 1.036), 12 of which are
radio-quiet quasars (RQQs) and 16 are radio-loud blazars. The subsample of
blazars consists of eight flat-spectrum radio-quasars (FSRQs) and eight BL Lac
objects. We find clear differences between the two samples. A majority of the
radio-loud AGN show moderate to high degrees of polarization, more than half of
them also show variability in polarization. There seems to be a dividing line
for polarization intranight variability at P~5 per cent over which all objects
vary in polarization. Only two out of 12 radio-quiet quasars show polarized
emission, both at levels of P<1 per cent. The lack of polarization intranight
variability in radio-quiet AGN points towards accretion instabilities being the
cause for intranight flux variability whereas the high duty cycle of
polarization variability in radio-loud objects is more likely caused by
instabilities in the jet or changes of physical conditions in the jet plasma.
