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

Abstract

Context. Due to its powerful radiation over the entire electromagnetic spectrum and its radio jet activity, the blazar 3C 273 offers the opportunity to study the physics of γ-ray emission from active galactic nuclei. Since a historically strong outburst in 2009, 3C 273 showed relatively weak emission in the γ-ray band over multiple years. However, recent Fermi-Large Area Telescope observations indicate elevated activity during 2015-2019. Aims. We aim at constraining the origin of the γ-ray outbursts towards 3C 273 and investigate their connection to the parsec-scale jet. Methods. We generate Fermi-LAT γ-ray light curves with multiple binning intervals and study the spectral properties of the γ-ray emission. Using a 3 mm ALMA light curve, we study the correlation between radio and γ-ray emission. Relevant activity in the parsec-scale jet of 3C 273 is investigated with 7 mm VLBA observations 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 mm-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 at 1.03 × 10 −7 ph cm −2 s −1 . A significant correlation between the γ-ray and mm-wavelength emission is found, with the radio lagging the γ-rays by about 105-112 days. The 43 GHz jet images reveal the known stationary features (i.e., the core, S 1, and S 2) 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 γ-ray outbursts. Conclusions. Our results support a parsec-scale origin for the observed γ-ray elevated activity, suggesting association with standing shocks in the jet.

Flux variability from ejecta in structured relativistic jets with large-scale magnetic fields

“…The number of knots observed is however strongly linked to the angular resolution and sensitivity of the VLBA. Another stationary knot very close to the core at ∼0.10−0.16 mas, noted A1, ST1, or S1 has been detected when observing the jet at 43.2 GHz (Jorstad et al 2005;Savolainen et al 2006;Lisakov et al 2017;Kim et al 2020).…”

Jet Properties of Compact Steep-Spectrum Sources and an Eddington-Ratio-Driven Unification Scheme of Jet Radiation in Active Galactic Nuclei

“…Excluding the four data points with L γ > 5 × 10 46 erg s −1 , the Pearson correlation analysis yields r = 0.45 and p = 0.09 for 4C 15.05. Recently, a transition from softer-when-brighter to harder-when-brighter is also observed in the monthly γ-ray flux-index plot of 3C 273 (Kim et al 2020). This transition may be due to a balance between acceleration and cooling of relativistic particles (Kim et al 2020) or a shift of the inverse Compton peak in the SED (Shah et al 2019).…”

Flare echos from relaxation shocks in perturbed relativistic jets

“…While their appearance is also possible in the absence of stationary components under certain conditions, interactions with the latter lead to a larger diversity of trailing components. Stationary components and their apparent drifting after interactions with a perturbation are observed in many sources with VLBI observations (Kim et al 2020;Lico et al 2022). The oscillation or drifting of stationary features can lead to additional emission counterparts and can help to explain observations as in the case of 3C 111.…”

“…Interactions between moving knots and standing knots have been proposed to give rise to MWL flares (Agudo et al 2012;Wehrle et al 2016;Kim et al 2020), which can be interpreted as shock-shock interactions. The typical time scale and temporal shape of flares are wavelength-dependent.…”