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

The 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. We use radio and $γ$-ray data obtained with ALMA, OVRO, SMA and Fermi/LAT to study the cross-correlation between $γ$-ray and multi-frequency radio light curves. Moreover, we employ VLBA observations at 43 GHz over a period of around nine years to study the parsec-scale jet kinematics. 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. We find two bright and compact newly ejected jet components that are likely associated with a high activity period visible in the $γ$-ray and radio light curves. The kinematic analysis of the VLBA observations leads to a maximum apparent jet speed of $β_{app}=19\pm10$ and an upper limit on the viewing angle of $φ$ < 4 deg. We determine the power law indices that are characterizing the jet geometry, brightness temperature distribution, and core shift to be $l=0.974\pm0.098$, $s=-3.31\pm0.31$, and $k_r=1.09\pm0.17$, which are 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 $2.6\,\mathrm{pc}\leq d_γ\leq20\,\mathrm{pc}$. Our derived 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 seed photons from the BLR.