Investigating the connection between gamma-ray activity and relativistic jet in 3C273 during 2015-2019

Due to its powerful radiation over the entire electromagnetic spectrum and its radio jet activity, the blazar 3C273 offers the opportunity to study the physics of $\gamma$-ray emission from active galactic nuclei. Since a historically strong outburst in 2009, 3C273 showed relatively weak emission in the gamma-ray band over multiple years. However, recent Fermi-Large Area Telescope observations indicate elevated activity during 2015-2019. We aim at constraining the origin of the gamma-ray outbursts towards 3C273 and investigate their connection to the parsec-scale jet. We generate Fermi-LAT gamma-ray light curves with multiple binning intervals and study the spectral properties of the gamma-ray emission. Using a 3-mm ALMA light curve, we study the correlation between radio and gamma-ray emission. Relevant activity in the parsec-scale jet of 3C273 is investigated with 7-mm VLBA observations obtained close in time to notable gamma-ray outbursts. We find two prominent gamma-ray outbursts in 2016 (MJD 57382) and 2017 (MJD 57883) accompanied by mm-wavelength flaring activity. The gamma-ray photon index time series show a weak hump-like feature around the gamma-ray outbursts. The monthly gamma-ray flux-index plot indicates a transition from softer-when-brighter to harder-when-brighter at $1.03\times10^{-7}\rm\,ph\,cm^{-2}\,s^{-1}$. A significant correlation between the gamma-ray and mm-wavelength emission is found, with the radio lagging the gamma-rays by about 105-112 days. The 43-GHz jet images reveal the known stationary features (i.e., the core, S1, and S2) in a region upstream of the jet. We find indication for a propagating disturbance and a polarized knot between the stationary components around the times of both gamma-ray outbursts.
Comment: 14 pages, 11 figures, 3 tables. To appear in A&A (submitted: 2020 January 10; accepted: 2020 March 5)