2011A&A...528L...3B

This article shows the first evidence ever of gravitational lensing phenomena in high energy gamma-rays. This evidence comes from the observation of an echo in the light curve of the distant blazar PKS 1830-211 induced by a gravitational lens system. Traditional methods for estimating time delays in gravitational lensing systems rely on the cross-correlation of the light curves from individual images. We used the 300MeV-30GeV photons detected by the Fermi-LAT instrument. It cannot separate the images of known lenses, so the observed light curve is the superposition of individual image light curves. The Fermi-LAT instrument has the advantage of providing long, evenly spaced, time series with very low photon noise. This allows us to use Fourier transform methods directly. A time delay between the two compact images of PKS 1830-211 has been searched for by both the autocorrelation method and the ``double power spectrum'' method. The double power spectrum shows a 4.2σ proof of a time delay of 27.1±0.6days, consistent with others' results.