Understanding BL Lacertae objects. Structural and 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 and 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. By analyzing the pc-scale jet morphology and its changes in 0735+178 we seek to understand the emission processes in BL Lac objects and to decipher their differences from quasars. 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 (2 cm/MOJAVE survey) performed at 23 epochs between 1995.27 and 2008.91. Multiple Gaussians were fitted to the derived VLBA data, to trace the kinematical and flux density evolution of the individual VLBI (Very Long Baseline Interferometry) components in the nuclear jet. We then compared the jet kinematics with the optical and radio light curves available for this BL Lac object and point out some striking correlations between the properties of the radio knots and the features in the light curves. 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. Conclusions. This is the third BL Lac object (after 1803+784 and 0716+714) analyzed by us, which reveals kinematic properties in the parsec-scale radio jet, which are atypical of quasars. For several years, the components in the jet are not found to be separating from the core but instead to be moving perpendicularly to the ridge line of the jet. In 0735+178 this unusual behavior has been particularly conspicuous. Curiously, however, fast apparent superluminal motion characteristic of quasars were present in this jet between the epochs ~1995 and ~2000. Thus we found for this BL Lac object a drastic change in the kinematics of the nuclear jet, i.e, transition from "typical superluminal" to an unusual "stationary" state. Interestingly, we found that this change is accompanied with a mode change in the nuclear jet's morphology. The long sequences of VLBA images reported here for this BL Lac object do not support the commonly assumed connection between radio flux-density outbursts and the ejection of new VLBI components. In contrast, the two mode changes seem correlated with the radio flares and are more likely an outcome of a changing viewing angle of the same knots. We propose therefore that all these observed features of the nuclear jet of 0735+178 and their correlation with the optical and radio flares might hold significant promise to better understand the basic difference between BL Lac objects and quasars.