The extreme blazar AO 0235+164 as seen by extensive ground and space radio observations

Abstract

Clues to the physical conditions in radio cores of blazars come from measurements of brightness temperatures as well as effects produced by intrinsic opacity. We study the properties of the ultra compact blazar AO 0235+164 with RadioAstron ground-space radio interferometer, multi-frequency VLBA, EVN and single-dish radio observations. We employ visibility modeling and image stacking for deriving structure and kinematics of the source, and use Gaussian process regression to find the relative multi-band time delays of the flares. The multi-frequency core size and time lags support prevailing synchrotron self absorption. The intrinsic brightness temperature of the core derived from ground-based VLBI is close to the equipartition regime value. In the same time, there is evidence for ultra-compact features of the size of less than 10 µas in the source, which might be responsible for the extreme apparent brightness temperatures of up to 10 14 K as measured by RadioAstron. In 2007-2016 the VLBI components in the source at 43 GHz are found predominantly in two directions, suggesting a bend of the outflow from southern to northern direction. The apparent opening angle of the jet seen in the stacked image at 43 GHz is two times wider than that at 15 GHz, indicating a collimation of the flow within the central 1.5 mas. We estimate the Lorentz factor Γ = 14, the Doppler factor δ = 21, and the viewing angle θ = 1.7• of the apparent jet base, derive the gradients of magnetic field strength and electron density in the outflow, and the distance between jet apex and the core at each frequency.

Significant core shift variability in parsec-scale jets of active galactic nuclei

“…Recently, core shift measurements have been applied to estimating the velocity of jet plasma flow (see e.g. Kutkin et al 2014;Lisakov et al 2017;Kutkin et al 2018a). These works take the ratio of the core shift magnitude and the time delay between flares at the same pair of frequencies as the flow velocity.…”

“…If flow velocities are underestimated, it may explain the extreme brightness temperatures observed e.g. by RadioAstron Gómez et al 2016;Kardashev et al 2017;Pilipenko et al 2018;Kutkin et al 2018b) and significantly alter various other estimates of jet parameters. Variability of the core position reported above can provide an independent way to estimate the plasma speed in the jet.…”

“…Core shift measurements are commonly used for making estimates of various geometrical and physical parameters of AGN jets (see, e.g . Lobanov 1998b;O'Sullivan & Gabuzda 2009;Kovalev et al 2008;Hada et al 2011;Kutkin et al 2014Kutkin et al , 2018bKravchenko et al 2016;Pushkarev et al 2018). Our analysis shows that core shift measured between two particular bands can vary substantially over time and that these variations are well connected to flares observed in compact jets.…”

“…Note that it does not require core shift measurements and can be done even for observations at a single frequency. In addition, as we show in Section 5.2, core shift variability measurements and the model we employ can be used to estimate the plasma flow velocity in the jet (see also on this subject Kutkin et al 2018a).…”

Detection statistics of the RadioAstron AGN survey

“…RadioAstron results on selected individual sources were presented earlier by Kovalev et al (2016); Edwards et al (2017); Pilipenko et al (2018); Kutkin et al (2018) with an emphasis on the AGN brightness issue. In this paper we discuss RadioAstron detection results for a complete VLBI-fluxdensity limited sample of bright AGN jets.…”