Constraining γ-ray dissipation site in gravitationally lensed quasar – PKS 1830−211

Abstract

Variable <ani:tex-math id="TM0001" notation="LaTeX">$/gamma$</ani:tex-math>-ray flares up to minute time-scales 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 <ani:tex-math id="TM0002" notation="LaTeX">$/gamma$</ani:tex-math>-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 centre. We have utilized 15 yr of Fermi–LAT <ani:tex-math id="TM0003" notation="LaTeX">$/gamma$</ani:tex-math>-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 <ani:tex-math id="TM0005" notation="LaTeX">$/gamma$</ani:tex-math>-ray emission zone, possibly within the radio core. The estimated time delay of approximately 20 d for the five flaring epochs was significantly shorter than previously estimated radio delays. This suggests that the <ani:tex-math id="TM0006" notation="LaTeX">$/gamma$</ani:tex-math>-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 <ani:tex-math id="TM0007" notation="LaTeX">$/gamma$</ani:tex-math>-ray emission zone originates from similar regions away from the site of radio dissipation. © 2025 The Author(s). Published by Oxford University Press on behalf of Royal Astronomical Society.