Increasing the Number of TeV Blazars with Parsec-Scale Kinematics

Abstract: We report on our observations of the parsec-scale radio jet structures of five blazars that have been detected by ground-based TeV gamma-ray telescopes. These five blazars all belong to the class of high-frequency peaked BL Lac objects (HBLs), which are the most common blazar type detected at the TeV energy range. Because of their relative faintness in the radio, these HBLs are not well represented in other radio blazar surveys. Our observations consist of five epochs of Very Long Baseline Array (VLBA) imaging from 2006 to 2009, of each of the five blazars 1ES 1101-232, Markarian 180, 1ES 1218+304, PG 1553+113, and H 2356-309, at frequencies from 5 to 22 GHz. Fundamental jet properties, including the apparent jet speeds, that can be measured from these multi-epoch series of VLBA images are presented and compared with other gamma-ray blazars. Confirming prior work, we find that the TeV HBLs have significantly slower apparent jet speeds than radio-selected blazars. Together with other radio properties of the HBL class, this suggests modest Lorentz factors in their parsec-scale radio jets. This in turn suggests some form of Lorentz factor gradient in these jets, since they are likely to have high Lorentz factors in their TeV-emitting regions. The study presented here approximately doubles the number of TeV HBLs with multi-epoch parsec-scale kinematic measurements.