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

Aims. This article shows the first evidence for gravitational lensing phenomena in high energy gamma-rays. This evidence comes from the observation of a gravitational lens induced echo in the light curve of the distant blazar PKS 1830-211. Methods. Traditional methods for the estimation of time delays in gravitational lensing systems rely on the cross-correlation of the light curves of the individual images. In this paper, we use 300 MeV-30 GeV photons detected by the Fermi-LAT instrument. The Fermi-LAT instrument cannot separate the images of known lenses. The observed light curve is thus the superposition of individual image light curves. The Fermi-LAT instrument has the advantage of providing long, evenly spaced, time series. In addition, the photon noise level is very low. This allows to use directly Fourier transform methods. Results. A time delay between the two compact images of PKS 1830-211 has been searched for both by the autocorrelation method and the "double power spectrum" method. The double 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). The relative uncertainty on the time delay estimation is reduced from 20% to 5%.