Multi-epoch VLBA Imaging of 20 New TeV Blazars: Apparent Jet Speeds

Abstract

We present 88 multi-epoch Very Long Baseline Array (VLBA) images (most at an observing frequency of 8 GHz), of 20 TeV blazars, all of the HBL class, that have not been previously studied at multiple epochs on the parsec scale. From these 20 sources, we analyze the apparent speeds of 43 jet components that are all detected at four or more epochs. As has been found for other TeV HBLs, the apparent speeds of these components are relatively slow. About two-thirds of the components have an apparent speed that is consistent (within 2σ) with no motion; and some of these components may be stationary patterns whose apparent speed does not relate to the underlying bulk flow speed. In addition, a superluminal tail to the apparent speed distribution of the TeV HBLs is detected for the first time, with eight components in seven sources having a 2σ lower limit on the apparent speed exceeding 1c. We combine the data from these 20 sources with an additional 18 sources from the literature to analyze the complete apparent speed distribution of all 38 TeV HBLs that have been studied with VLBI at multiple epochs. The highest 2σ apparent speed lower limit considering all sources is 3.6c. This suggests that bulk Lorentz factors of up to about 4, but probably not much higher, exist in the parsec-scale radio emitting regions of these sources, consistent with estimates obtained in the radio by other means such as brightness temperatures. This can be reconciled with the high Lorentz factors estimated from the high-energy data if the jet has velocity structures consisting of different emission regions with different Lorentz factors. In particular, we analyze the current apparent speed data for the TeV HBLs in the context of a model with a fast central spine and a slower outer layer.

A parsec-scale wobbling jet in the high-synchrotron peaked blazar PG 1553+113

“…The source shows a compact core region with a northeast jet structure detected up to ∼ 1.5 mas (∼ 7.8 pc in linear size) at 15 GHz and ∼ 1 mas (∼ 5.2 pc) at 43 GHz. This morphology is in agreement with 15 GHz MOJAVE images, as well as with the 22 and 43 GHz results by Piner & Edwards (2018). The jet position As shown in the lower image in Fig.…”

“…We obtain an average value for δ of ∼ 1.4, which is a typical value for HSP objects as obtained from VLBI observations (e.g. Giroletti et al 2004;Lico et al 2012;Piner & Edwards 2018). Such a low value for the Doppler factor further supports our conclusion that the Doppler boosting modulation alone cannot account for the observed variability.…”

MOJAVE. XVII. Jet Kinematics and Parent Population Properties of Relativistically Beamed Radio-loud Blazars

“…Figure 6. Maximum apparent jet speed versus synchrotron peak frequency for jets in the MOJAVE survey, as well as those in the survey of Piner & Edwards (2018). Upper limit values are denoted by downward arrows.…”

Opacity, variability, and kinematics of AGN jets

“…The physical nature of these emission knots remains unclear (Zensus et al 1995), while it is widely assumed that these regions of enhanced emission are somehow associated with shock waves in the relativistic plasma flow (e.g., Blandford & Königl 1979, Marscher & Gear 1985. Most of the knots move in a jet direction with super-or sub-luminal apparent speeds, while stationary components also seem to be a common occurrence (Rani et al 2015, Jorstad et al 2017, especially in BL Lac type objects (Piner & Edwards 2018). A wide range of component speeds is often observed within a single source, which makes their relation to the velocity of the underlying plasma flow unclear (e.g., Lister et al 2016, and references therein).…”