Non-radial motion in the TeV blazar S5 0716+714

Abstract

Context. Flat-spectrum radio sources often show a core-jet structure on pc-scales. Individual jet components reveal predominantly outward directed motion. For the BL Lac object S5 0716+714 conflicting apparent velocities have been reported in the literature. This object is an intra-day variable source and suited to investigate a possible correlation between kinematic properties and flux-density variability on different timescales. Aims. We study the kinematics in the pc-scale jet of S5 0716+714 to determine the apparent speeds of the jet components based on a much improved data set. In addition, we search for correlations between the radio flux-density light curves and the morphological changes detected along the VLBI jet. Methods. We (re-)analyze 50 VLBI observations obtained with the VLBA at 5 different frequencies (5-43 GHz) between 1992.73 and 2006.32. The data have been parameterized using circular Gaussian components. We analyze the jet component motion in detail taking care not only to account for motion in the radial but also in the orthogonal direction. We study the evolution of the jet ridge line and investigate the spectral properties of the individual components. We search for correlations between radio band light curves and the kinematic properties of the jet components. Results. We present an alternative kinematic scenario for jet component motion in S5 0716+714. We present evidence for the apparent stationarity of jet components (with regard to their core separation) with time. Jet components, however, do seem to move significantly non-radially with regard to their position angle and in a direction perpendicular to the major axis of the jet. We discuss a possible correlation between the long-term radio flux-density variability and apparent jet component motions. Conclusions. In S5 0716+714 an alternative motion scenario is proposed. With regard to the core separation, rather stationary components can fit the VLBI observations well. A new model to explain the observed motion with regard to the position angle is required. Based on the correlation between the longterm radio flux-density variability and the position angle evolution of a jet component, we conclude that a geometric contribution to the origin of the long-term variability might not be negligible. Subluminal motion has been reported for most of the TeV blazars. Our analysis also confirms this finding for the case of S5 0716+714. This result increases the number of TeV blazars showing apparent subluminal motion to 7.

Understanding BL Lacertae objects

“…A similar phenomenon -rather stationary components relative to the core, but moving perpendicular to the jet ridge line -was studied by us in the two prominent BL Lac objects, 1803+784 (Britzen et al 2009) and 0716+714 (Britzen et al 2009). Over a time of ∼20 years we do find rather slow apparent motion or stationarity for nearly all of the components in the pc-scale jet of 1803+784.…”

Multi-frequency VLBA study of the blazar S5 0716+714 during the active state in 2004

“…Note that the kinematics of the outer jet are consistent with the results of Britzen et al (2009). 0.64 ± 0.14 0.55 ± 0.11 10.6 ± 2.2 C3 1.9 ± 0.4 0.62 ± 0.41 11.8 ± 8.0 C4 3.2 ± 0.4 0.44 ± 1.4 8.6 ± 26.5 C5 5.7 ± 0.9 -1.37 ± 2.7 -26.4 ± 52.2 C6 11.6 ± 0.6 -0.27 ± 1.2 5.2 ± 23.6 Notes.…”

“…The long-term kinematics of this source have been extensively studied, and two mutually exclusive models have been suggested: one implies relatively fast outward motion of the superluminal components in the jet with speeds ranging from 5c to 16c ("fast scenario", Bach et al 2005), and the other claims the jet to be stationary, with significant non-radial ("precession") motion of the jet as a whole ("stationary scenario" Britzen et al 2009). Both these scenarios are based on the reanalysis of historical VLBI observations that span the years 1992-2001 ("fast") and 1992-2006 ("stationary") at frequencies from 5 to 43 GHz.…”

Intra-day variability observations of S5 0716+714 over 4.5 years at 4.8 GHz

“…On the other hand, considering that the position angle of the 0716+714 jet is close to the anisotropy angle derived from our ISS model fit, it is plausible that the anisotropic scattering is caused by a sourceintrinsic anisotropy. Because the inner-jet position angle is oscillating from ∼10 • to ∼35 • following a 5.7 ± 0.5-year long-term variability cycle in the total flux density of 0716+714 (Raiteri et al 2003;Fan et al 2007;Britzen et al 2009), the anisotropy angle may vary accordingly. This would also contribute to an increase of the χ 2 r of the annual modulation fit.…”