Research Note

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

¹ ISDC Data Centre for Astrophysics, Ch. d’Ecogia 16, 1290 Versoix Switzerland
e-mail: andrii.neronov@unige.ch
² Geneva Observatory, Ch. des Maillettes 51, 1290 Sauverny, Switzerland
³ APC, 10 rue Alice Domon et Leonie Duquet, 75205 Paris Cedex 13, France
⁴ Institute for Nuclear Research RAS, 60th October Anniversary prosp. 7a, Moscow 117312, Russia
⁵ Service de Physique Theorique, Universite Librede Bruxelles, CP225, blv. du Triomphe, 1050 Bruxelles, Belgium

Received: 8 October 2010
Accepted: 18 November 2010

Abstract

We analyze the gamma-ray halo around stacked AGNs reported by Ando & Kusenko (2010, ApJ, 722, L39). First, we show that the angular distribution of γ-rays around the stacked AGNs is consistent with the angular distribution of the γ-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 γ-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.