Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Parsec-scale Magnetic-Field Structures in HEAO-1 BL Lacs   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.

21
The categories of this paper are: adap-org
25
The categories of this paper are: patt-sol
36
The categories of this paper are: cmp-lg
26
The categories of this paper are: bayes-an
5
The categories of this paper are: gr-qc
13
The categories of this paper are: quant-ph
22
The categories of this paper are: chao-dyn
20
The categories of this paper are: acc-phys
28
The categories of this paper are: alg-geom
33
The categories of this paper are: plasm-ph
32
The categories of this paper are: atom-ph
4
The categories of this paper are: cond-mat
14
The categories of this paper are: math-ph
30
The categories of this paper are: q-alg
7
The categories of this paper are: hep-th
35
The categories of this paper are: mtrl-th