Gamma-ray Time Delay and Magnification Ratio in the Gravitationally-Lensed Blazar PKS 1830-211

TL;DR

This study measures the gamma-ray time delay and magnification ratio in the gravitationally lensed blazar PKS 1830-211 using advanced time-series analysis, providing insights into lensing effects and jet structure.

Contribution

It introduces a novel approach to estimate magnification ratios and employs improved techniques to measure gamma-ray time delays in PKS 1830-211.

Findings

Measured gamma-ray time delay of 20.26 days with high precision.

Estimated gamma-ray flux ratio between lens components is less than 1.8.

Found no convincing evidence of microlensing effects.

Abstract

We present the characterization of macrolensing properties of the gravitationally lensed system PKS 1830-211, utilizing data from the Fermi Large Area Telescope. While at gamma-rays we can not spatially resolve the lensed images, a macrolensing-induced time pattern is expected in the blazar's lightcurve, resulting from the delay between variable gamma-ray components originating from its two brightest lensed images. Compared to our previous study, here we employ high-quality lightcurves coupled with prolonged outburst activity, and improved time-series techniques. Analyzing six independent data segments, we identified a delay of 20.26 +/- 0.62 days (statistical and stochastic uncertainty), with a chance detection probability at the 2.5 x $10^{-5}$ level (post-trial). We also present a novel approach to the magnification ratio estimate based on a comparison with simulated data. Our work suggests that the gamma-ray flux ratio between the two main lens components is $\mu_{\gamma} \lesssim$ 1.8. We do not observe convincing evidence of microlensing effects, as previously claimed. The measured gamma-ray time delay is in 2-sigma tension with radio-based estimates, suggesting either distinct emission sites, underestimated radio uncertainties, or gamma-ray production in a region opaque to radio. Our study highlights the potential of well sampled lightcurves and advanced time-series techniques to distinguish true lensing-induced delays from stochastic variability. When combined with improved radio constraints and refined lens models, PKS 1830-211 and similar sources constitute promising systems for time-delay cosmography, offering new insights into both jet structure and cosmological parameters.