Very Long Baseline Polarimetry of BL Lacertae

Abstract

We present the results of an extensive observing campaign designed to study the evolution of parsecscale radio structure in the nucleus of the AGN galaxy BL Lacertae. The observations spanned 17 epochs from 1994.7 to 1998.3. The VLBA observations, made at regular intervals at 15, 22, and 43 GHz, show the ejection and evolution of four highly polarized superluminal components (denoted S7ÈS10). The trajectories of all components were signiÐcantly curved, with the slowest component (S8) exhibiting the most bending. All four components had nearly constant apparent speeds, at least between 1 and 3 mas separation from the core. Extrapolation of the core-component separation to a zero spacing epoch suggests that they may have been created in pairs, with fast and slow counterparts. Each superluminal component was moderately linearly polarized at all observed frequencies. The electric vector position angle (EVPA) was frequency independent but changed with core-component separation for all four components, with total EVPA rotation varying from 30¡ (S7) to 80¡ (S8). There was no preferred EVPA orientation with respect to the jet axis when components were young, i.e., close to the core. However, three of the four componentsÏ EVPAs rotated to within 20¡ of the jet direction at later epochs, consistent with emission from transverse shocks. The core was nearly unpolarized at 15 GHz, except when a new superluminal component was just emerging. At higher frequencies (22 and 43 GHz) core fractional polarization was low with a quadratic frequency dependence. We applied HardeeÏs model of helical twisting on an adiabatically expanding jet to explain the observed bent component trajectories. By searching the parameter space of allowed helical geometries, we found a best-Ðt set of parameters with Ðxed opening and line of sight angles for all four components. The preferred helical geometry was described by a line of sight angle #^9¡ and a jet half-opening angle (^2¡. Individual component trajectories were Ðtted by varying initial conditions at the throat of the jet and the initial helical wavelength scale. Although the position along the jet and apparent speed are consistent with the helical model, the predicted polarization position angles are not in good agreement. We derived physical properties in the emission regions by assuming that they are due to optically thin synchrotron radiative shocks within a underlying relativistic Ñow. Using the observed fractional polarization and jet-interjet intensity ratio along with the helix-line of sight angle as constraints, we found that slower components (S8, S10) are consistent with forward shocks, while faster components (S7, S9) could be either forward or reverse shocks. Derived shock compression ratios vary from weak (k D 0.5È0.8) for slow component S8 to moderately strong (k D 0.0È0.5) for fast component S9. As of epoch 1998.2, there have been no unusual features in the parsec-scale radio structure following the large opticalÈgamma-ray Ñare at epoch 1997.6.

Polarization and Structure of Relativistic Parsec-Scale AGN Jets

“…This type of behaviour (predominance of longitudinal polarization at larger distances) is seen in some cases (Denn et al 2000;Gabuzda & Gómez 2001).…”

Analysis of the long-term polarization behaviour of BL Lac

“…For BL Lac this direction (θ = 114 • ) is nearly orthogonal to the direction of the movement of VLBI components in the 1980s, 188 • −196 • (Mutel et al 1990). Furthermore, the magnetic field is even closer to perpendicular to the jet axis on 1-2 milliarsecond scales of 195 • −206 • (Denn et al 2000) as one can expect if the optical radiation arises in shocks in the jet relatively close to the central engine (Marscher & Gear 1985). Such behaviour of the polarization at radio wavelengths is not unusual and can be explained by models involving transverse shocks moving along the jet (Hughes et al 1985).…”

WEBT multiwavelength monitoring and XMM-Newton observations of BL Lacertae in 2007–2008

“…Indeed, VLBA/VLBI studies of the jet structure in AGNs have revealed bent jet morphologies that are suggestive of streaming motions along a helical path (see e.g. Lister 2001) or that the magnetic field may present a helical geometry (Gabuzda et al 2004); this is also true for BL Lacertae (Tateyama et al 1998;Denn et al 2000;Marscher et al 2008;O'Sullivan & Gabuzda 2009, see also Stirling et al 2003). Moreover, a rotating helical path in a curved jet was invoked by Villata et al (2009) to explain the optical and radio behaviour of BL Lacertae in the last forty years, in particular the alternation of enhanced and suppressed optical activity, accompanied by hard and soft radio events, respectively.…”