Multi-Epoch VLBA Imaging of Twenty New TeV Blazars: Apparent Jet Speeds

TL;DR

This study uses multi-epoch VLBA imaging to measure apparent jet speeds in 20 TeV blazars, revealing generally slow motions with some superluminal components, and suggests jet velocity structures with different Lorentz factors.

Contribution

First multi-epoch VLBA imaging of 20 TeV blazars, providing new measurements of jet apparent speeds and evidence for velocity structures in their jets.

Findings

Most jet components are stationary or slow-moving.

Detected superluminal components with speeds exceeding 1c.

Estimated bulk Lorentz factors are up to about 4.

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 two-sigma) 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 two-sigma 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 two-sigma 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.