New limits on the density of the extragalactic background light in the optical to the far-infrared from the spectra of all known TeV blazars

Daniel Mazin, Martin Raue

Published 2007-01-24, updated 2007-06-27Version 3

Aims: We derive limits on the EBL density from the energy spectra of distant sources of very high energetic gamma-rays (VHE gamma-rays). Methods: VHE gamma-rays are attenuated by the photons of the EBL via pair production, which leaves an imprint in the measured spectra from distant sources. In this paper we present a new method to derive constraints on the EBL. Hereby, we use only very basic assumptions about TeV blazar physics and no pre-defined EBL model, but rather a large number of generic shapes constructed from a grid in EBL density vs. wavelength. In our study we utilize spectral data from all known TeV blazars, making this the most complete study so far. Results: We derive limits on the EBL for three individual TeV blazar spectra (Mkn 501, H1426+428, 1ES1101-232) and for all spectra combined. Combining the results from individual spectra leads to significantly stronger constraints over a wide wavelength range from the optical (~ 1 micron) to the far-infrared (~ 80 microns). The limits are only a factor of 2 to 3 above the absolute lower limits derived from source counts. In the mid-infrared our limits are the strongest constraints derived from TeV blazar spectra so far. A high density of the EBL around 1 micron, reported by direct detection experiments, can be excluded. Conclusions: Our results can be interpreted in two ways: (i) The EBL is almost resolved by source counts, leaving only little room for additional components by e.g. the first stars or (ii) the assumptions about the underlying physics are not valid, which would require substantial changes in the standard emission models of TeV blazars.