Letters to the Editor

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

¹ DSM/IRFU, CEA/Saclay, 91191 Gif-sur-Yvette, France
e-mail: glicens@cea.fr
² Nicolaus Copernicus Astronomical Center, Warszawa, Poland
e-mail: abarnack@camk.edu.pl

Received: 19 November 2010
Accepted: 24 January 2011

Abstract

Aims. 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.

Methods. 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 300 MeV–30 GeV 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.

Results. 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 ± 0.6 days, consistent with others’ results.