Cheung, C. C.C. C.CheungLarsson, S.S.LarssonScargle, J. D.J. D.ScargleCiprini, StefanoStefanoCiprini2020-09-172020-09-172014-02-01https://hdl.handle.net/20.500.13025/3638The Astrophysical Journal Letters, Volume 782, Issue 2, article id. L14, <NUMPAGES>7</NUMPAGES> pp. (2014).Using data from the Fermi Large Area Telescope (LAT), we report the first clear gamma-ray measurement of a delay between flares from the gravitationally lensed images of a blazar. The delay was detected in B0218+357, a known double-image lensed system, during a period of enhanced gamma-ray activity with peak fluxes consistently observed to reach >20-50 its previous average flux. An auto-correlation function analysis identified a delay in the gamma-ray data of 11.46 0.16 days (1sigma) that is ~1 day greater than previous radio measurements. Considering that it is beyond the capabilities of the LAT to spatially resolve the two images, we nevertheless decomposed individual sequences of superposing gamma-ray flares/delayed emissions. In three such ~8-10 day-long sequences within a ~4 month span, considering confusion due to overlapping flaring emission and flux measurement uncertainties, we found flux ratios consistent with ~1, thus systematically smaller than those from radio observations. During the first, best-defined flare, the delayed emission was detailed with a Fermi pointing, and we observed flux doubling timescales of ~3-6 hr implying as well extremely compact gamma-ray emitting regions.galaxies activegamma rays galaxiesgravitational lensing strongquasars individual B0218+35710.1088/2041-8205/782/2/L14http://adsabs.harvard.edu/abs/2014ApJ...782L..14C2014ApJ...782L..14C