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Description
The powerful radiation over the entire electromagnetic spectrum and its radio jet activity of the blazar 3C 273 offer the opportunity of studying the physics of {gamma}-ray emission from active galactic nuclei. Since the historically strong outburst in 2009, 3C 273 showed relatively weak emission in the {gamma}-ray band over several years. However, recent Fermi-Large Area Telescope observations indicate higher activity during 2015-2019. We constrain the origin of the {gamma}-ray outbursts toward 3C 273 and investigate their connection to the parsec-scale jet. We generated Fermi-LAT {gamma}-ray light curves with multiple binning intervals and studied the spectral properties of the {gamma}-ray emission. Using a 3mm ALMA light curve, we studied the correlation between radio and {gamma}-ray emission. The relevant activity in the parsec-scale jet of 3C 273 was investigated with 7 mm VLBA observations that were 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 millimeter-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 states at 1.03x10^-7^ph/cm^2^/s. A significant correlation between the {gamma}-ray and millimeter-wavelength emission is found, and the radio lags the {gamma}-rays by about 105-112 days. The 43GHz jet images reveal the known stationary features (i.e., the core, S1, and S2) in a region upstream of the jet. We find an indication for a propagating disturbance and a polarized knot between the stationary components at about the times of the two {gamma}-ray outbursts. Our results support a parsec-scale origin for the observed higher {gamma}-ray activity, which suggests that this is associated with standing shocks in the jet.
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