Speaker
Description
The gravitationally lensed blazar PKS 1830-211 underwent a historically bright and unusually long-duration gamma-ray flaring episode in 2019/2021 with daily fluxes exceeding > $10^{-6}$ ph/cm$^2$/s for ~400 days, and daily peak fluxes (> $10^{-5}$ ph/cm$^2$/s) exceeding all prior flares observed by the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope in the first 15 years of operation. We analyzed six bright flaring episodes (~hundreds of days long) independently using an auto-correlation function analysis, improved by a new methodology for detrending of the gamma-ray light curve, i.e. the removal of the underlying low-frequency trend due to the red noise stochastic components. We identify a significant delay in the gamma-ray data of the order of 20 days. The value is consistent over the different observational epochs, and is attributable to the gravitational lensing effect. The results and uncertainties presented here account for stochastic variability of the blazar for the first time and, in contrast to prior works that considered the earliest fainter flaring intervals and reported disparate time delay values in gamma rays, we here consider improvements in the reliability of the LAT lightcurves, and benefits from improvements enabled by the present LAT calibrations. In this contribution, we present these new findings, and discuss how the possible discrepancy between the time delay measured at gamma-rays (20 days) and the one derived from radio observations (~25 days) may be reconciled, e.g. as a probe of different emission regions responsible for the gamma rays and radio emission, and/or microlensing effects.
