A1 Refereed original research article in a scientific journal
Pinpointing the location of the γ-ray emitting region in the FSRQ 4C+01.28
Authors: Rösch, F.; Kadler, M.; Ros, E.; Ricci, L.; Gurwell, M. A.; Hovatta, T.; Macdonald, N. R.; Readhead, A. C. S.
Publisher: EDP Sciences
Publication year: 2025
Journal: Astronomy and Astrophysics
Article number: A143
Volume: 704
ISSN: 0004-6361
eISSN: 1432-0746
DOI: https://doi.org/10.1051/0004-6361/202556231
Publication's open availability at the time of reporting: Open Access
Publication channel's open availability : Open Access publication channel
Web address : https://doi.org/10.1051/0004-6361/202556231
Self-archived copy’s web address: https://research.utu.fi/converis/portal/detail/Publication/508259446
Self-archived copy's licence: CC BY
Self-archived copy's version: Publisher`s PDF
Aims. The flat-spectrum radio quasar (FSRQ) 4C +01.28 is a bright and highly variable radio and gamma-ray emitter. We aim to pinpoint the location of the gamma-ray emitting region within its jet in order to derive strong constraints on gamma-ray emission models for blazar jets. Methods. We use radio and gamma-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 gamma-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 gamma-ray emitting region, we use a model in which outbursts shown in the gamma-ray and radio light curves are produced when moving jet components pass through the gamma-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 gamma-ray and different radio light curves. The kinematic analysis of the VLBA observations leads to a maximum apparent jet speed of beta(app) = 19 +/- 10 and an upper limit on the viewing angle of phi less than or similar to 4 degrees. 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 gamma-ray light curve. We pinpoint the location of the gamma-ray emitting region with respect to the jet base to the range of 2.6 pc <= d(gamma) <= 20 pc. Conclusions. Our observational limits places the location of gamma-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.
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We thank the anonymous referee for their helpful comments and suggestions which improved the manuscript. We thank S. G. Jorstad, A. P. Marscher and Z. R. Weaver for helpful discussions and comments on the kinematic analysis. F. R., M. K. and L. R. acknowledge support from the Deutsche Forschungsgemeinschaft (DFG, grants 434448349 and 443220636 [FOR5195: Relativistic Jets in Active Galaxies]). T. H. was supported by Academy of Finland projects 317383, 320085, 345899, and 362571. This study makes use of the following ALMA data: ADS/JAO.ALMA#2011.0.00001.CAL. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), MOST and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. This study makes use of VLBA data from the VLBA-BU Blazar Monitoring Program (BEAM-ME and VLBA-BU-BLAZAR; http://www.bu.edu/blazars/BEAM-ME.html), funded by NASA through the Fermi Guest Investigator Program. The VLBA is an instrument of the National Radio Astronomy Observatory, which is a facility of the National Science Foundation operated by Associated Universities, Inc. The Submillimeter Array is a joint project between the Smithsonian Astrophysical Observatory and the Academia Sinica Institute of Astronomy and Astrophysics and is funded by the Smithsonian Institution and the Academia Sinica. We recognize that Maunakea is a culturally important site for the indigenous Hawaiian people; we are privileged to study the cosmos from its summit.
