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Aims. This article shows the first evidence for gravitational lensingphenomena in high energy gamma-rays. This evidence comes from the observationof a gravitational lens induced echo in the light curve of the distant blazarPKS 1830-211. Methods. Traditional methods for the estimation of time delays ingravitational lensing systems rely on the cross-correlation of the light curvesof the individual images. In this paper, we use 300 MeV-30 GeV photons detectedby the Fermi-LAT instrument. The Fermi-LAT instrument cannot separate theimages of known lenses. The observed light curve is thus the superposition ofindividual image light curves. The Fermi-LAT instrument has the advantage ofproviding long, evenly spaced, time series. In addition, the photon noise levelis very low. This allows to use directly Fourier transform methods. Results. Atime delay between the two compact images of PKS 1830-211 has been searched forboth by the autocorrelation method and the "double power spectrum" method. Thedouble power spectrum shows a 3 {\sigma} evidence for a time delay of 27.5$\pm$ 1.3 days, consistent with the result from Lovell et al. (1998). Therelative uncertainty on the time delay estimation is reduced from 20% to 5%.

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