Speaker
Description
The flat spectrum radio quasar OP 313 showed intense $\gamma$-ray emission from November 2023 to March 2024, as observed by the Large Area Telescope on board the Fermi Gamma-ray Space Telescope. From that event a large number of follow-up campaigns at all wavelengths started, confirming the increase of the source activity from the radio to very high energy (VHE) bands. Remarkably, it also led to the first detection of the VHE emission from OP313, making it also the most distant Active Galactic Nuclei detected at VHE $\gamma$-ray band.
In this work, we investigate this flaring period and perform a multi-wavelength analysis covering 15 years of Fermi-LAT observations, from August 2008 to March 2024. From the $\gamma$-ray light-curve study, we identified different periods of high-state activity. These are compared with the data available from other facilities. From the comparison results the trend of the light-curves in $\gamma$-rays, X-rays, UV, and optical shows an increasing flux starting from the end of 2021. Before then, the source is characterized by small flares marginally visible in the $\gamma$-ray light-curve. Instead, looking at the radio light-curves, the common trend is that the source was characterized by high fluxes from the beginning of our analysis in 2008 and, then, it shows a decreasing trend until 2019, when it started to increase again. To unveil the reason behind this different behavior and understand how radiation in different wavelengths is connected in OP 313, we focused on the study of the 7 highest $\gamma$-ray flares and of the jet's kinematics. From 3 of the 7 selected flares, we were able to produce the hysteresis pattern which gave us information about the dominating mechanism that cools down the particles in the jet. Then, from the comparison of the Spectral Energy Distributions (SEDs) of the 7 flares, we noticed that the first one is less Compton Dominated than the others. This means that different photon fields from inside and outside the jet are responsible for the detected high-energy emission. The evidence of the external seed photons' presence for the Compton scattering process was obtained through the modelization of the SEDs of 2 flares that happened at a 2-year distance, one in 2022 and one in 2024. Using a one-zone leptonic model for both of them, it results that in the recent one, the External Compton scattering of the photons coming from the dusty torus gains prominence, accompanied by a reduced synchrotron flux.
From the analysis of the visibilities of the Monitoring Of Jets in Active galactic nuclei with VLBA Experiments and VLBA-BU-BLAZAR projects from 2008 to 2024, we found new knots arising from the core of OP 313 which can be responsible for the intense flaring emission detected since 2022. From the kinematic study of these new components, we derived their epoch of ejection, the Doppler, and Lorentz factors, and the viewing angle. All these parameters quantify the jet's beaming properties.