Non-thermal cosmic backgrounds from blazars: the contribution to the CMB, X-ray and γ-ray backgrounds

¹ ASI Science Data Center, ASDC c/o ESRIN, via G. Galilei, 00044 Frascati, Italy e-mail: paolo.giommi@asi.it
² Agenzia Spaziale Italiana, Unitá Osservazione dell'Universo, viale Liegi 26, 00198 Roma, Italy
³ INAF - Osservatorio Astronomico di Roma via Frascati 33, 00040 Monteporzio, Italy
⁴ Universitá di Roma “ Tor Vergata” and INFN sez. Roma 2, Italy

Received: 11 May 2005
Accepted: 8 August 2005

Abstract

We present a new assessment of the contribution of the blazar population to the extragalactic background radiation across the electromagnetic spectrum. Our calculations rely on deep blazar radio counts that we have derived by combining several radio and multi-frequency surveys. We show that blazar emission integrated over cosmic time gives rise to a considerable broad-band non-thermal cosmic background that in some parts of the electromagnetic spectrum dominates the extragalactic brightness.
We confirm that blazars are the main discrete contributors to the Cosmic Microwave Background (CMB), where we estimate that their integrated emission causes an apparent temperature increase of 5-50 μK in the frequency range 50-250 GHz. The CMB primordial fluctuation spectrum is contaminated starting at multipole , in the case of a completely random source distribution, or at lower values if spatial clustering is present. We estimate that well over one hundred-thousand blazars will produce a significant signal in the maps of the upcoming Planck CMB anisotropy mission. Because of a tight correlation between the microwave and the X-ray flux, these sources are expected to be X-ray emitters with flux larger than a few 10^-15 erg cm^-2 s^-1 in the soft X-ray band. A large fraction of the foreground sources in current and near-future CMB anisotropy maps could therefore be identified and removed using a multi-frequency approach, provided that a sufficiently deep all-sky X-ray survey will become available in the near future.
We show further that blazars are a major constituent of all high energy extragalactic backgrounds. Their contribution is expected to be 11-12% at X-ray frequencies and possibly 100% in the ∼0.5-50 MeV band. At higher energies (MeV) the estimated blazar collective emission, obtained by extrapolating their integrated micro-wave flux to the γ-ray band using the SED of EGRET detected sources, overestimates the extragalactic background by a large factor, thus implying that not only blazars dominate the γ-ray sky but also that their average duty cycle at these frequencies must be rather low. Finally, we find that blazars of the HBL type may produce a significant amount of flux at TeV energies.