Fermi-LAT Detection of Gravitational Lens Delayed Gamma-ray Flares from Blazar B0218+357

TL;DR

This paper reports the first gamma-ray measurement of a gravitational lens delay in a blazar, B0218+357, revealing a delay of about 11.46 days and indicating extremely compact gamma-ray emitting regions.

Contribution

It provides the first gamma-ray detection of a gravitational lens delay in a blazar, using Fermi-LAT data to analyze flare sequences and measure the delay with high precision.

Findings

Measured a gamma-ray delay of 11.46 days in B0218+357

Detected flux doubling timescales of 3-6 hours indicating compact emission regions

Flux ratios during flares differ from radio observations, suggesting different emission regions or processes.

Abstract

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 times its previous average flux. An auto-correlation function analysis identified a delay in the gamma-ray data of 11.46 +/- 0.16 days (1 sigma) 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 hrs implying as well extremely compact gamma-ray emitting regions.