Identifying changing jets through their radio variability

Abstract

Context. Supermassive black holes can launch highly relativistic jets with velocities reaching Lorentz factors of as high as Γ > 50. How the jets accelerate to such high velocities and where along the jet they reach terminal velocity are open questions that are tightly linked to their structure as well as their launching and dissipation mechanisms. Aims. Changes in the beaming factor along the jets could potentially reveal jet acceleration, deceleration, or bending. We aim to (1) quantify the relativistic effects in multiple radio frequencies and (2) study possible jet velocity–viewing angle variations at parsec scales. Methods. We used the state-of-the-art code Magnetron to model light curves from the University of Michigan Radio Observatory and the Metsähovi Radio Observatory’s monitoring programs in five frequencies covering about 25 years of observations in the 4.8 to 37 GHz range for 61 sources. We supplement our data set with high-frequency radio observations in the 100–340 GHz range from ALMA, CARMA, and SMA. For each frequency we estimate the Doppler factor which we use to quantify possible changes in the relativistic effects along the jets. Results. The majority of our sources do not show any statistically significant difference in their Doppler factor across frequencies. This is consistent with constant velocity in a conical jet structure, as expected at parsec scales. However, our analysis reveals 17 sources where relativistic beaming changes as a function of frequency. In the majority of cases, the Doppler factor increases towards lower frequencies. Only 1253–053 shows the opposite behavior. By exploring their jet properties we find that the jet of 0420–014 is likely bent across the 4.8–340 GHz range. For 0212+735, the jet is likely parabolic, and still accelerating in the 4.8–37 GHz range. We discuss possible interpretations for the trends found in the remaining sources.

Optical circular polarization of blazar S4 0954+65 during high linear polarized states

“…We used the results of Magnetron to compute the maximum observed brightness temperature distribution, which we compared to the intrinsic maximum brightness temperature of T int,max = 2.78×10 11 K . We find a Doppler factor of δ = 11.6 +11.1 −5.5 , which is consistent with previous estimates in the literature (Liodakis et al 2017(Liodakis et al , 2021. The uncertainties on δ are not statistical, but rather express the range of possible values over the ∼15 years of radio observations.…”

A VLBI Proper Motion Analysis of the Recoiling Supermassive Black Hole Candidate Mrk 1018

“…It is known that flat-spectrum radio sources exhibit flux density variability over timescales from days to years (Hovatta et al 2007;Chen et al 2013;Sadler et al 2014). Interstellar scintillation (Walker 1998;Said et al 2020) and variations in the Doppler boosting factor (Liodakis et al 2021;Kosogorov et al 2022) are two processes that may cause the observed flux density variability. For flat-spectrum radio sources, in particular the blazar subclass, this variability is often associated with flaring activity, and it is a major source of misidentification of Peaked Spectrum (PS) objects (Kovalev et al 2002;Jauncey et al 2003;Tinti et al 2005;Torniainen et al 2005;Orienti et al 2010), i.e., the radio spectrum becomes peaked where it is expected to be flat.…”