Understanding BL Lac objects Structural & kinematic mode changes in the BL Lac object PKS 0735+178

Abstract

Context. We present evidence that parsec-scale jets in BL Lac objects may be significantly distinct in kinematics from their counterparts in quasars. We argued this previously for the BL lac sources 1803+784 and 0716+714, report here a similar pattern for another well-known BL Lac object, PKS 0735+178, whose nuclear jet is found to exhibit kinematics atypical of quasars. Aims. A detailed study of the jet components' motion reveals that the standard AGN paradigm of apparent superluminal motion does not always describe the kinematics in BL Lac objects. We study 0735+178 here to augment and improve the understanding of the peculiar motions in the jets of BL Lac objects as a class. Methods. We analyzed 15 GHz VLBA (Very Long Baseline Array) observations (2cm/MOJAVE survey) performed at 23 epochs between 1995.27 and 2008.91. Results. We found a drastic structural mode change in the VLBI jet of 0735+178, between 2000.4 and 2001.8 when its twice sharply bent trajectory turned into a linear shape.We further found that this jet had undergone a similar transition sometime between December 1981 and June 1983. A mode change, occurring in the reverse direction (between mid-1992 and mid-1995) has already been reported in the literature. These structural mode changes are found to be reflected in changed kinematical behavior of the nuclear jet, manifested as an apparent superluminal motion and stationarity of the radio knots. In addition, we found the individual mode changes to correlate in time with the maxima in the optical light curve. The last two transitions occurred before a (modest) radio flare. The behavior of this pc-scale jet appears to favor a scenario involving non-ballistic motions of the radio knots, produced by the precession of a continuous jet within the ambient medium.

Accepted for publication in A&A (Abstract reduced for astro-ph)

References in corpus (9)

• MOJAVE: Monitoring of Jets in AGN with VLBA Experiments. V. Multi-epoch VLBA Images
• Optical Intra-day Variability in Blazars
• A Multi-epoch VLBI Survey of the Kinematics of Caltech-Jodrell Bank Flat-Spectrum Sources, Part II: Analysis of the Kinematics
• Ten-year optical monitoring of PKS 0735+178: historical comparison, multiband behaviour and variability timescales
• The kinematics in the pc-scale jets of AGN The case of S5 1803+784
• Non-radial motion in the TeV blazar S5 0716+714 The pc-scale kinematics of a BL Lac Object
• Unusual optical quiescence of the classical BL Lac object PKS 0735+178 on intranight time scale
• The milliarcsecond-scale jet of PKS 0735+178 during quiescence
• The non-ballistic superluminal motion in the plane of the sky

Cited by in corpus (18)

• Kinematics of Parsec-Scale Jets of Gamma-Ray Blazars at 43~GHz within the VLBA-BU-BLAZAR Program
• A multi-messenger study of the blazar PKS 0735+178: a new major neutrino source candidate
• On the source of Faraday rotation in the jet of the radio galaxy 3C120
• Intra-night optical variability of core dominated radio quasars: the role of optical polarization
• Studies of the Jet in BL Lacertae. II. Superluminal Alfvén Waves
• Precession-induced Variability in AGN Jets and OJ 287
• Multi-wavelength variability study of the classical BL Lac object PKS 0735+178 on timescales ranging from decades to minutes
• Rapid optical variability of TeV blazars
• Multi-band Variability Analysis of Mrk 421
• The variability of blazars throughout the electromagnetic spectrum
• The parsec-scale jet of PKS 1749+096
• Deconstructing blazars: A different scheme for jet kinematics in flat-spectrum AGN
• Gamma-rays in flat-spectrum AGN: Revisiting the fast jet hypothesis with the CJF sample
• Correlations between the peak flux density and the position angle of inner-jet in three blazars
• Multi-epoch leptohadronic modeling of neutrino source candidate blazar PKS 0735+178
• Using bootstrap to assess uncertainties of VLBI results I. The method and image-based errors
• The non-ballistic superluminal motion in the plane of the sky-II
• AGN jets under the microscope: A divide?