Letter to the Editor

The spectrum of 1ES0229 + 200 and the cosmic infrared background

¹ NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA e-mail: stecker@milkyway.gsfc.nasa.gov
² James Madison University, Harrisonberg, VA 22807, USA

Received: 10 October 2007
Accepted: 9 November 2007

Abstract

Aims.We reexamine the implications of the recent HESS observations of the blazar 1ES0229+200 for constraining the extragalactic mid-infrared background radiation.

Methods.We examine the effect of γ-ray absorption by the extragalactic infrared radiation on predicted intrinsic spectra for this blazar and compare our results with the observational data.

Results.We find agreement with our previous results on the shape of the infrared spectral energy distribution, contrary to the recent assertion of the HESS group. Our analysis indicates that 1ES0229+200 has a very hard intrinsic spectrum with a spectral index between 1.1 ± 0.3 and 1.5 ± 0.3 in the energy range between ~0.5 TeV and ~15 TeV.

Conclusions.Under the assumptions that (1) the models of Stecker et al. (2006, ApJ, 648, 774) as derived from numerous detailed infrared observations are reasonable, and (2) spectral indexes in the range 1 < Γ < 1.5 are obtainable from relativistic shock acceleration under the astrophysical conditions extant in blazar flares (Stecker et al. 2007, ApJ, 667, L29), the fits to the observations of 1ES0229+200 using our previous infrared spectral energy distributions are consistent with both the infrared and γ-ray observations. Our analysis presents evidence indicating that the energy spectrum of relativistic particles in 1ES0229+200 is produced by relativistic shock acceleration, producing an intrinsic γ-ray  spectrum with index 1 < Γ < 1.5 and with no evidence of a peak in the spectral energy distribution up to energies ~15 TeV.