Parsec-Scale Structure and Kinematics of Faint TeV HBLs

B Piner,Philip Edwards

doi:10.3390/galaxies4040044

Abstract

We present new multi-epoch Very Long Baseline Array (VLBA) observations of a set of TeV blazars drawn from our VLBA program to monitor all TeV-detected high-frequency peaked BL Lac objects (HBLs) at parsec scales. Most of these sources are faint in the radio, so they have not been well observed with VLBI by other surveys. Our previous measurements of apparent jet speeds in TeV HBLs showed apparent jet speeds that were subluminal or barely superluminal, suggesting jets with velocity structures at the parsec-scale. Here we present apparent jet speed measurements for eight new TeV HBLs, which for the first time show a superluminal tail to the apparent speed distribution for the TeV HBLs.

Highlights

At TeV energies (1012 eV), three orders of magnitude higher than those studied by satellite-based detectors, gamma-ray astronomy is conducted with ground based telescopes such as H.E.S.S., VERITAS and MAGIC
Multi-epoch studies of these jets over the course of several years allow the apparent speeds of the jet components to be determined
Our on-going Very Long Baseline Array (VLBA) monitoring of the growing number of TeV gamma-ray emitting high-frequency peaked BL Lac objects (HBLs) has revealed that the distribution of peak apparent jet speeds in these sources extends to moderate superluminal speeds, ∼6c, but the majority display subluminal speeds, in contrast with the distribution for other classes of active galactic nuclei [5]

Summary

Introduction

At TeV energies (1012 eV), three orders of magnitude higher than those studied by satellite-based detectors, gamma-ray astronomy is conducted with ground based telescopes such as H.E.S.S., VERITAS and MAGIC. Over 175 TeV gamma-ray sources have been catalogued (http://tevcat.uchicago.edu/), with over one third of these being extragalactic objects. The majority of these (46 of 69) are classified as HBL (High-frequency–peaked BL Lac) objects, for which the synchrotron peak of the Spectral Energy. The only way to directly obtain information on the parsec-scale structure of these blazar jets is by imaging the radio sources using the technique of Very Long Baseline Interferometry (VLBI). Many HBLs are fainter at radio wavelengths (typically tens of milli-janskys—see Table 1) than the more powerful quasars and BL Lac objects, and are not included in VLBI monitoring programs such as MOJAVE [5] and TANAMI [6]. Properties that can be measured from VLBI images—the apparent jet speed, radio core brightness temperature, core dominance, and jet-to-counter-jet brightness ratio—provide information on fundamental properties of the jet, such as the bulk Lorentz factor and viewing angle

Results

Discussion

Conclusion