Locating the γ-ray emitting region in the quasar 4C +01.28

F. Rösch*, M. Kadler, E. Ros, M. Gurwell, T. Hovatta, M. Kreter, N. R. MacDonald, A. C.S. Readhead

*Corresponding author for this work

Research output: Contribution to journalConference articleScientificpeer-review

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Determining the location of γ-ray emission in blazar jets is a challenging task. Pinpointing the exact location of γ-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 γ-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 provided by the Boston University (BU) Blazar Monitoring Program 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 γ-ray telescope and at various radio frequencies. A cross-correlation analysis reveals the radio light curves systematically lag behind the γ-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 distance of the γ-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.

Original languageEnglish
Article number001
JournalProceedings of Science
Publication statusPublished - 2021
MoE publication typeA4 Conference publication
EventEuropean VLBI Network Symposium and Users Meeting - Virtual, Online
Duration: 12 Jul 202114 Jul 2021


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