Constraining $\gamma$-ray dissipation site in gravitationally lensed quasar -- PKS 1830$-$211

TL;DR

This study uses 15 years of Fermi-LAT gamma-ray data and machine learning to constrain the location of gamma-ray emission in the gravitationally lensed quasar PKS 1830-211, revealing it is closer to the central engine than the radio emission zone.

Contribution

The paper introduces a machine learning approach to estimate time delays in gravitationally lensed gamma-ray flares, providing new constraints on emission site locations in a high-redshift blazar.

Findings

Consistent time delay of ~20 days across flaring states.

Gamma-ray emission zone is closer to the central engine than the radio core.

Linear relationship between lag and magnification observed.

Abstract

Variable $\gamma$-ray flares upto minute timescales reflect extreme particle acceleration sites. However, for high-redshift blazars, the detection of such rapid variations remains limited by current telescope sensitivities. Gravitationally lensed blazars serve as powerful tools to probe $\gamma$-ray production zones in distant sources, with time delays between lensed signals providing crucial insights into the spatial distribution of emission regions relative to the lens's mass-weighted center. We have utilized 15 years of Fermi-LAT $\gamma$-ray data from direction of PKS 1830$-$211 to understand the origin of flaring high-energy production zone at varying flux states. To efficiently estimate the (lensed) time delay, we used a machine learning-based tool - the Gaussian Process regression algorithm, in addition to - Autocorrelation function and Double power spectrum. We found a consistent time delay across all flaring activity states, indicating a similar location for the $\gamma$-ray emission zone, possibly within the radio core. The estimated time delay of approximately 20 days for the five flaring epochs was significantly shorter than previously estimated radio delays. This suggests that the $\gamma$-ray emission zone is closer to the central engine, in contrast to the radio emission zone, which is expected to be much farther away. A linear relationship between lag and magnification has been observed in the identified source and echo flares. Our results suggest that the $\gamma$-ray emission zone originates from similar regions away from the site of radio dissipation.