Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Constraining the magnetic field in the parsec-scale jets of the
  brightest Fermi blazars with multifrequency VLBI observations   The spatially resolved broad-band spectroscopy with Very Long Baseline
Interferometry (VLBI) is one of the few methods that can probe the physical
conditions inside blazar jets. We report on measurements of the magnetic field
strength in parsec-scale radio structures of selected bright Fermi blazars,
based on fitting the synchrotron spectrum to VLBA images made at seven
frequencies in a 4.6 -- 43.2 GHz range. Upper limits of B <= 10^-2 -- 10^2 G
(observer's frame) could be placed on the magnetic field strength in 13
sources. Hard radio spectra (-0.5 <= a <= +0.1, S_nu ~ nu^a) observed above the
synchrotron peak may either be an indication of a hard energy spectrum of the
emitting electron population or result from a significant inhomogeneity of the
emitting region.

25
The categories of this paper are: patt-sol
2
The categories of this paper are: math
4
The categories of this paper are: cond-mat
30
The categories of this paper are: q-alg
35
The categories of this paper are: mtrl-th
32
The categories of this paper are: atom-ph
5
The categories of this paper are: gr-qc
8
The categories of this paper are: hep-ex
29
The categories of this paper are: funct-an
19
The categories of this paper are: eess
20
The categories of this paper are: acc-phys
26
The categories of this paper are: bayes-an
23
The categories of this paper are: solv-int
10
The categories of this paper are: q-bio
22
The categories of this paper are: chao-dyn
16
The categories of this paper are: stat