VLBI observations of seven BL Lac objects from RGB sample

TL;DR

This study uses high-resolution VLBI observations and Doppler factor estimates to analyze the jet properties and intrinsic emission characteristics of seven BL Lac objects, revealing correlations with luminosity and viewing angle.

Contribution

It provides new insights into the intrinsic synchrotron peak frequency's dependence on radio power and viewing angle, based on detailed VLBI data and Doppler factor calculations.

Findings

Doppler factors are larger in LBLs than in IBLs and HBLs.

An anti-correlation exists between 408 MHz luminosity and intrinsic peak frequency.

A positive correlation is found between viewing angle and intrinsic peak frequency.

Abstract

We present EVN observations of seven BL Lac objects selected from the RGB sample. To investigate the intrinsic radiation property of BL Lac objects, we estimated the Doppler factor with the VLA or MERLIN core and the total 408 MHz luminosity for a sample of 170 BL Lac objects. The intrinsic (comoving) synchrotron peak frequency was then calculated by using the estimated Doppler factor. Assuming a Lorentz factor of 5, the viewing angle of jets was constrained. The high-resolution VLBI images of seven sources all show a core-jet structure. We estimated the proper motions of three sources with the VLBI archive data, and find that the apparent speed increases with the distance of components to the core for all of them. In our BL Lacs sample, the Doppler factor of LBLs is systematically larger than that of IBLs and HBLs. We find a significant anti-correlation between the total 408 MHz luminosity and the intrinsic synchrotron peak frequency. However, the scatter is much larger than for the blazar sequence. Moreover, we find a significant positive correlation between the viewing angle and the intrinsic synchrotron peak frequency. The BL Lac objects show a continuous distribution on the viewing angle. While LBLs have a smaller viewing angle than that of IBLs and HBLs, IBLs are comparable to HBLs. We conclude that the intrinsic synchrotron peak frequency is not only related to the intrinsic radio power (though with a large scatter), but also to the viewing angle for the present sample.