Intranight polarization variability in radio-loud and radio-quiet AGN

TL;DR

This study investigates intranight polarization variability in AGN, revealing significant differences between radio-loud and radio-quiet types, with implications for understanding their emission mechanisms.

Contribution

It provides the first comparative analysis of intranight polarization variability in radio-loud and radio-quiet AGN, highlighting distinct polarization behaviors and their likely physical origins.

Findings

Radio-loud AGN often show moderate to high polarization and variability.

Radio-quiet quasars rarely show polarization or variability.

A polarization level of ~5% marks a threshold for variability.

Abstract

(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.