Cascading on extragalactic background light

¹ Laboratoire Leprince-Ringuet, UMR 7638 CNRS, École Polytechnique, 91128 Palaiseau, France e-mail: Guillaume.Dubus@obs.ujf-grenoble.fr
² Institut d'Astrophysique de Paris, UMR 7095 CNRS, Université Pierre & Marie Curie, 98bis Bd. Arago, 75014 Paris, France
³ Laboratoire d'Astrophysique de Grenoble, UMR 5571 CNRS, Université Joseph Fourier, BP 53, 38041 Grenoble, France

Received: 8 November 2006
Accepted: 21 April 2007

Abstract

Context.High-energy γ-rays propagating in the intergalactic medium can interact with background infrared photons to produce e⁺ e^- pairs, resulting in the absorption of the intrinsic γ-ray spectrum. TeV observations of the distant blazar 1ES 1101-232 were thus recently used to put an upper limit on the infrared extragalactic background light density.

Aims.The created pairs can upscatter background photons to high energies, which in turn may pair produce, thereby initiating a cascade. The pairs diffuse on the extragalactic magnetic field (EMF) and cascade emission has been suggested as a means for measuring its intensity. Limits on the IR background and EMF are reconsidered taking into account cascade emissions.

Methods.The cascade equations are solved numerically. Assuming a power-law intrinsic spectrum, the observed 100 MeV–100 TeV spectrum is found as a function of the intrinsic spectral index and the intensity of the EMF.

Results.Cascades emit mainly at or below 100 GeV. The observed TeV spectrum appears softer than for pure absorption when cascade emission is taken into account. The upper limit on the IR photon background is found to be robust. Inversely, the intrinsic spectra needed to fit the TeV data are uncomfortably hard when cascade emission makes a significant contribution to the observed spectrum. An EMF intensity around 10^-8 nG leads to a characteristic spectral hump in the GLAST band. Higher EMF intensities divert the pairs away from the line-of-sight and the cascade contribution to the spectrum becomes negligible.