Investigating the connection between γ-ray activity and the relativistic jet in 3C 273 during 2015−2019

Abstract

Context. 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 γ-ray emission from active galactic nuclei. Since the historically strong outburst in 2009, 3C 273 showed relatively weak emission in the γ-ray band over several years. However, recent Fermi-Large Area Telescope observations indicate higher activity during 2015−2019. Aims. We constrain the origin of the γ-ray outbursts toward 3C 273 and investigate their connection to the parsec-scale jet. Methods. We generated Fermi-LAT γ-ray light curves with multiple binning intervals and studied the spectral properties of the γ-ray emission. Using a 3 mm ALMA light curve, we studied the correlation between radio and γ-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 γ-ray outbursts. Results. We find two prominent γ-ray outbursts in 2016 (MJD 57382) and 2017 (MJD 57883) accompanied by millimeter-wavelength flaring activity. The γ-ray photon index time series show a weak hump-like feature around the γ-ray outbursts. The monthly γ-ray flux–index plot indicates a transition from softer-when-brighter to harder-when-brighter states at 1.03 × 10−7 ph cm−2 s−1. A significant correlation between the γ-ray and millimeter-wavelength emission is found, and the radio lags the γ-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 an indication for a propagating disturbance and a polarized knot between the stationary components at about the times of the two γ-ray outbursts. Conclusions. Our results support a parsec-scale origin for the observed higher γ-ray activity, which suggests that this is associated with standing shocks in the jet.