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

Abstract

Intranight polarization variability in active galactic nuclei (AGN) has not been studied extensively so far. Studying the variability in polarization makes it possible 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. Each AGN was observed for a time-span of ∼4 h in the R band to measure polarization and variability. Using statistical methods, we determine duty cycles for polarized emission and polarization intranight variability. 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. We did not find clear correlations between the strength of the variability and the redshift or degree of polarization. Only two out of 12 RQQs 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. We were able to constrain the time-scale of the detected variations to >4 h. Further studies of intranight polarization variability will be necessary to reveal exact physical conditions behind this phenomenon.

Polarimetry of optically selected BL Lacertae candidates from the SDSS

“…We then computed P Q and P U using standard formulae (e.g. Villforth et al 2009). In this phase, we checked that there were no spurious objects inside the measurement aperture.…”

The first IRAM/PdBI polarimetric millimeter survey of active galactic nuclei

“…Variability of AGN polarization is well-known in the cm/radio regime (e.g., Aller et al 1985) and has been probed in the mm/radio regime for a few selected sources (e.g., Jorstad et al 2005) as well as in the optical (e.g., Villforth et al 2009). Our variability analysis (see Eqs.…”

Flux and Polarization Variability of OJ 287 during the Early 2016 Outburst

“…Sillanpaa et al (1991), based on the observations carried over six nights, found anticorrelation between flux and polarization variations which they explained as a result of highly rotating plasma inside a relativistic jet. However, Villforth et al (2009) did not find any clear correlation between flux and polarization. From long term (year time scale) photopolarimetric observations, Efimov et al (2002) noticed rapid continuous rotation of the position angle of about 4.92 degrees/day in clockwise direction suggesting a helical magnetic field jet structure.…”

“…Intra-night polarization variability (INPV) of OJ 287 has been studied earlier by Villforth et al (2009) who found about 16 % polarization. On the nights of 05, 06 and 10 April 2016, we have sufficient data points in R-band to characterize the INPV of OJ 287.…”

“…Among them, a precessing binary black hole in which the secondary black hole affects the accretion disk of the primary is more favorable than others as it predicts more accurately the timing of the major outburst (Sundelius et al 1997;Valtonen & Ciprini 2012). OJ 287 has also been studied for polarization variability (Shakhovskoi & Efimov 1977;Sillanpaa et al 1991Sillanpaa et al , 1992Valtaoja et al 2000; Pursimo et al 2000;Efimov et al 2002;Villforth et al 2009). Sillanpaa et al (1991), based on the observations carried over six nights, found anticorrelation between flux and polarization variations which they explained as a result of highly rotating plasma inside a relativistic jet.…”