Pinpointing the location of the γ-ray emitting region in the FSRQ 4C +01.28

¹ Julius-Maximilians-Universität Würzburg, Fakultät für Physik und Astronomie, Institut für Theoretische Physik und Astrophysik, Lehrstuhl für Astronomie, Emil-Fischer-Str. 31, D-97074 Würzburg, Germany
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³ Center for Astrophysics | Harvard & Smithsonian, Cambridge, MA 02138, USA
⁴ Finnish Centre for Astronomy with ESO (FINCA), University of Turku, FI-20014 Turku, Finland
⁵ Aalto University Metsähovi Radio Observatory, Metsähovintie 114, FI-02540 Kylmälä, Finland
⁶ Aalto University Department of Electronics and Nanoengineering, PL 15500, FI-00076 Aalto, Finland
⁷ The University of Mississippi, Department of Physics and Astronomy, Oxford, MS 38677, USA
⁸ Owens Valley Radio Observatory, California Institute of Technology, Pasadena, CA 91125, USA

^⋆ Corresponding author: florian.roesch@uni-wuerzburg.de

Received: 3 July 2025
Accepted: 25 September 2025

Abstract

Aims. The flat-spectrum radio quasar (FSRQ) 4C +01.28 is a bright and highly variable radio and γ-ray emitter. We aim to pinpoint the location of the γ-ray emitting region within its jet in order to derive strong constraints on γ-ray emission models for blazar jets.

Methods. We use radio and γ-ray monitoring data obtained with the Atacama Large Millimeter/submillimeter Array (ALMA), the Owens Valley Radio Observatory (OVRO), the Submillimeter Array (SMA), and the Large Area Telescope on board the Fermi Gamma-ray Space Telescope (Fermi/LAT) to study the cross-correlation between γ-ray and multifrequency radio light curves. Moreover, we employ Very Long Baseline Array (VLBA) observations at 43 GHz over a period of around nine years to study the parsec-scale jet kinematics of 4C +01.28. To pinpoint the location of the γ-ray emitting region, we use a model in which outbursts shown in the γ-ray and radio light curves are produced when moving jet components pass through the γ-ray emitting and the radio core regions.

Results. We find two bright and compact newly ejected jet components that are likely associated with a high activity period visible in the Fermi/LAT γ-ray and different radio light curves. The kinematic analysis of the VLBA observations leads to a maximum apparent jet speed of β_app = 19 ± 10 and an upper limit on the viewing angle of ϕ ≲ 4°. Furthermore, we determine the power law indices that are characterizing the jet geometry, brightness temperature distribution, and core shift to be l = 0.974 ± 0.098, s = −3.31 ± 0.31, and k_r = 1.09 ± 0.17, respectively, which are all in agreement with a conical jet in equipartition. A cross-correlation analysis shows that the radio light curves follow the γ-ray light curve. We pinpoint the location of the γ-ray emitting region with respect to the jet base to the range of 2.6 pc ≤ d_γ ≤ 20 pc.

Conclusions. Our observational limits places the location of γ-ray production in 4C +01.28 beyond the expected extent of the broad-line region (BLR) and therefore challenges blazar-emission models that rely on inverse Compton up-scattering of BLR seed photons.