Parsec-scale Faraday rotation distribution in the BL Lac object 1803+784

Abstract

We present the results of VLBI polarization observations of the BL Lac object 1803+784 made simultaneously at 22, 15, 8 and 5 GHz in 1997 April, and quasi-simultaneously at 1.6 and 5 GHz in 1998 July. These data have yielded the total intensity, polarization, spectral-index, and rotation-measure distributions on scales from less than 1 mas to about 30 mas from the core (from a projected distance of about 1-150 pc at the redshift of 1803+784). There is an enhancement in the rotation measure for the VLBI core, clearly due to plasma in the immediate vicinity of the AGN. The rotation measures 10-30 mas from the core demonstrate that the Galactic contribution to the total observed rotation measures is equal to the previously reported integrated rotation measure (-61 rad m(-2) ). After subtracting this Galactic contribution and multiplying by (1 +z )(2) , the rotation measure for the VLBI core in the source rest frame is about 500 rad m(-2), similar to the VLBI core rotation measures reported previously for BL Lac and several quasars

The parsec-scale distributions of intensity, linear polarization and Faraday rotation in the core and jet of Mrk 501 at 8.4-1.6 GHz

“…There is a clear systematic gradient in the RM across the VLBI jet, extending from the core region to a distance of about 30 mas from the core. This confirms the previous detection of such a RM gradient based on observations at 5, 8 and 15 GHz (Gabuzda et al 2004) and provides new support for the hypothesis that a helical magnetic field is associated with the jet of Mrk 501.…”

“…This is among the first VLBI polarization studies of AGN at frequencies as low as 1.6 and 2.2 GHz (e.g. Gabuzda & Chernetskii 2003; Pushkarev et al 2005; Hallahan & Gabuzda 2008).…”

VLBA polarization observations of BL Lac objects and passive elliptical galaxies

“…Moreover, while VLBI studies of quasars have revealed non‐uniform rotation measure distribution on the parsec scale (Taylor 1998, 2000; Nan et al 1999), the situation is not clear in BL Lac objects. OJ 287 shows no substantial rotation measure intrinsic to the BL Lac object (Gabuzda & Gómez 2001), while intrinsic rotation measure is found in the core of BL Lac itself (Reynolds, Cawthorne & Gabuzda 2001), and 1803+784 (Gabuzda & Chernetskii 2003).…”

The origin of intrinsic variability of intraday variable sources

“…It has been observed and studied with different VLBI arrays at a range of different frequencies (e.g., Eckart et al 1986Eckart et al , 1987Witzel et al 1988;Charlot 1990;Strom & Biermann 1991;Fey et al 1996;Gabuzda 1999;Gabuzda & Cawthorne 2000;Gabuzda & Chernetskii 2003;Ros et al 2000Ros et al , 2001Britzen et al 2005aBritzen et al ,b, 2008). …”