No evidence for gamma-ray halos around active galactic nuclei resulting from intergalactic magnetic fields

TL;DR

This study investigates gamma-ray halos around AGNs and finds no evidence that intergalactic magnetic fields cause such halos, attributing observed effects to instrumental artifacts instead.

Contribution

It provides a detailed analysis showing that previously reported gamma-ray halos are likely due to instrumental effects rather than intergalactic magnetic fields.

Findings

Gamma-ray distribution around AGNs matches that of a point source.

Observed excess is due to instrumental effects, not astrophysical halos.

No evidence supports gamma-ray halos caused by intergalactic magnetic fields.

Abstract

We analyze the gamma-ray halo around stacked AGNs reported in Ap.J.Lett., 2010, 722, L39. First, we show that the angular distribution of gamma-rays around the stacked AGNs is consistent with the angular distribution of the gamma-rays around the Crab pulsar, which is a point source for Fermi/LAT. This makes it unlikely that the halo is caused by an electromagnetic cascade of TeV photons in the intergalactic space. We then compare the angular distribution of gamma-rays around the stacked AGNs with the point-spread function (PSF) of Fermi/LAT and confirm the existence of an excess above the PSF. However, we demonstrate that the magnitude and the angular size of this effect is different for photons converted in the front and back parts of the Fermi/LAT instrument, and thus is an instrumental effect.