Pinpointing the location of the <i>γ</i> -ray emitting region in the FSRQ 4C +01.28

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.