Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Locating the $\gamma$-ray emitting region in the quasar 4C +01.28   Determining the location of $\gamma$-ray emission in blazar jets is a
challenging task. Pinpointing the exact location of $\gamma$-ray production
within a relativistic jet can place strong constraints on our understanding of
high-energy astrophysics and astroparticle physics. We present a study of the
radio- and $\gamma$-bright flat-spectrum radio quasar (FSRQ) 4C +01.28 (PKS
B1055+018) in which we try to pinpoint the emission site of several prominent
GeV flares. This source shows prominent high-amplitude broadband variability on
time scales ranging from days to years. We combine high-resolution VLBI
observations and multi-band radio light curves over a period of around nine
years. We can associate two bright and compact newly ejected jet components
with bright flares observed by the Fermi/LAT $\gamma$-ray telescope and at
various radio frequencies. A cross-correlation analysis reveals the radio light
curves systematically lag behind the $\gamma$-rays. In combination with the jet
kinematics as measured by the VLBA, we use these cross-correlations to
constrain a model in which the flares become observable at a given frequency
when a plasma component passes through the region at which the bulk energy
dissipation takes place at that frequency. We derive a lower limit of the
location of the $\gamma$-ray emitting region in 4C +01.28 of several parsecs
from the jet base, well beyond the expected extent of the broad-line region.
This observational limit challenges blazar-emission models that rely on the
broad-line region as a source of seed photons for inverse-Compton scattering.

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The categories of this paper are: q-alg
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The categories of this paper are: eess
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The categories of this paper are: atom-ph
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The categories of this paper are: chem-ph
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The categories of this paper are: patt-sol
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The categories of this paper are: nlin
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The categories of this paper are: hep-ex
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The categories of this paper are: hep-ph
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The categories of this paper are: cs
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The categories of this paper are: alg-geom
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The categories of this paper are: nucl-th
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The categories of this paper are: mtrl-th
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The categories of this paper are: acc-phys
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The categories of this paper are: hep-lat
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The categories of this paper are: funct-an
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The categories of this paper are: q-bio