Parsec-scale Magnetic-Field Structures in HEAO-1 BL Lacs

TL;DR

This study uses VLBI polarization imaging to compare magnetic field structures in high-energy-peaked and low-energy-peaked BL Lac objects, revealing differences in magnetic field orientation and suggesting intrinsic distinctions.

Contribution

It provides the first detailed polarization images of HBLs, highlighting differences in magnetic field orientation compared to LBLs and proposing intrinsic physical differences.

Findings

HBLs have lower core polarization than LBLs.

HBL jets tend to have longitudinal magnetic fields, unlike LBLs.

Differences may be due to intrinsic properties like black hole spin.

Abstract

We present very long baseline interferometry polarization images of an X-ray selected sample of BL Lacertae objects belonging to the first High Energy Astronomy Observatory (HEAO-1) and the ROSAT-Green Bank (RGB) surveys. These are primarily high-energy-peaked BL Lacs (HBLs) and exhibit core-jet radio morphologies on pc-scales. They show moderately polarized jet components, similar to those of low-energy-peaked BL Lacs (LBLs). The fractional polarization in the unresolved cores of the HBLs is, on average, lower than in the LBLs, while the fractional polarizations in the pc-scale jets of HBLs and LBLs are comparable. However a difference is observed in the orientation of the inferred jet magnetic fields -- while LBL jets are well-known to preferentially exhibit transverse magnetic fields, the HBL jets tend to display longitudinal magnetic fields. Although a `spine-sheath' jet velocity structure, along with larger viewing angles for HBLs could produce the observed magnetic field configuration, differences in other properties of LBLs and HBLs, such as their total radio power, cannot be fully reconciled with the different-angle scenario alone. Instead it appears that LBLs and HBLs differ intrinsically, perhaps in the spin rates of their central black holes.