The Faraday screen near the nucleus of the CSS quasar 3C 138

Abstract

Abstract. Compact Steep Spectrum (CSS) sources are known to show strong optical line emission (Gelderman & Whittle 1994) which is generally considered to be due to the interaction between the radio jet and the ISM. This argument is strengthened by the HST observations of Axon et al. (2000) which show this emission is frequently associated with the radio jet. Previous observations of the CSS quasar 3C 138 (Cotton et al. 1997) have shown that the bulk of the jet has very low amounts of Faraday rotation whereas the inner jet shows very high Faraday rotation (≈−5000 rad m −2 ). The inner jet is dominated by moving components which allows observing the fine scale of the Faraday screen as the components move behind it. We report the results of such a monitoring campaign which indicated that there is structure in the Faraday screen on the pc and possibly sub pc scale. Motions of the polarized component slowed dramatically during the course of these observations, limiting the region probed. The observations also strongly suggest that the polarized emission from the inner jet is seen through limited holes in a dense Faraday screen. In this case, the observed variations in Faraday rotation are possibly the result of jet-ISM interactions.

VLBA polarimetric observations of the CSS quasar 3C 147

“…This would explain the high Faraday rotation that we observe. The overall properties of 3C 147 are similar to those found in 3C 286 (Jiang et al 1996;Cotton et al 1997), and possibily 3C 138 (Cotton et al 2003b) (although both of these sources are highly polarised with low/absent RM) whose interpretation presents similar challenges.…”

“…-The extended structure to the southwest of the brightest region, which we propose to be a backflow tail, has a smooth brightness distribution; the structure is rather slim and is similar to the lobes in thin FR II sources, such as 3C 284 and 3C 341. On the small scale, it resembles the main "jet" of 3C 138 (e.g., Cotton et al 2003b), which is not a jet, since the current interpretation of the source postulates that the easternmost and brightest structure is indeed a hot spot. -The size of the various components and the width-to-length ratio of the brightest region can be more easily reconciled with a hot spot rather than the initial part of a jet (i.e., the core), since this would require substantial recollimation to maintain a small opening angle along the entire structure to the southeast.…”

“…CSSs for which mas-scale RM distributions have been derived are even rarer. We note here the cases of OQ 172 (Udomprasert et al 1997), 3C 216 (Venturi & Taylor 1999), 0548 + 165 and 1524 − 136 (Mantovani et al 2002), 3C 43 and 3C 454 (Cotton et al 2003c), and 3C 138 (Cotton et al 2003b). …”

Radio properties of CSSs and GPSs

“…∼ 2000 rad m −2 , Cotton et al 2003a; in the inner jet of 3C138, RM s.f. ∼ 5000 rad m −2 , Cotton et al 2003b; in SE lobe of 3C277.1, RM s.f. ∼ 1700 rad m −2 , Cotton et al 2006); d) when a source is resolved there is a mild tendency for the intrinsic magnetic field to be parallel to the jet axis …”

“…3C43, Cotton et al 2003a). In 3C138 Cotton et al (1997Cotton et al ( , 2003b probed the irregularities/holes in the Faraday screen by studying the temporal changes of the inner-jet rotation measure (RM) observed as the jet was advancing in the local ISM (Fig. 3).…”