Vol. 528
In section 1. Letters

First evidence of a gravitational lensing-induced echo in gamma rays with Fermi LAT

The distant quasar PKS 1830-211 is known to be lensed by a z=0.89 foreground galaxy, and it is split into radio in two images with an Einstein ring. Intrinsic flux variations of the quasar are seen in the two images with a time delay caused by the different optical paths in the two images. Here, for the first time, a gravitational lensing phenomenon is reported in high-energy gamma-rays: 300 MeV-30 GeV photons detected by the Fermi-LAT instrument. The two images are not spatially resolved, and the observed light curve is thus the superposition of individual image light curves. The long time series observed has allowed the time delay to be determined by the autocorrelation and "double power spectrum" methods. It is found to be 27.5 +/- 1.3 days, consistent with radio results.