Investigating the connection between gamma-ray activity and relativistic jet in 3c 273 during 2015-2019

The blazar 3C 273 is an obvious candidate to explore the nature of the $\gamma$-rays due to its powerful radiation over the entire electromagnetic spectrum and the radio jet activity. Since the historical outburst in 2009, 3C 273 has been showing relatively weak emission at $\gamma$-ray band for a long time. However, recent Fermi-Large Area Telescope observations indicate elevated activity during 2015--2019. The aim is to study the origin of $\gamma$-ray outbursts towards 3C 273 and investigate its connection to the parsec scale jet. We generate Fermi-LAT $\gamma$-ray light curves in multiple binning intervals and study spectral properties of the $\gamma$-ray emission. Using the 3 mm ALMA light curve, we study the correlation between radio and $\gamma$-ray emission. Relevant activity in the parsec scale jet of 3C 273 is investigated with the usage of 7 mm VLBA observations near the time of the 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 indices 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 $\times$ $10^{-7}$ 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 indications of the propagating disturbance and a polarized knot between the stationary components around the time of both the $\gamma$-ray outbursts. Our results support a parsec-scale origin for the observed $\gamma$-ray elevated activity, suggesting association with the standing shocks in the jet.