Estimation of the Extragalactic Background Light using TeV Observations of BL~Lacs

Understand

The very high energy (VHE) gamma ray spectral index of high energy peaked blazars correlates strongly with its corresponding redshift whereas no such correlation is observed in the X-ray or the GeV bands.

• We attribute this correlation to a result of photon-photon absorption of TeV photons with the extragalactic background light (EBL) and utilizing this, we compute the allowed flux range for the EBL, which is independent of previous estimates.
• The observed VHE spectrum of the sources in our sample can be well approximated by a power-law, and if the de-absorbed spectrum is also assumed to be a power law, then we show that the spectral shape of EBL will be $\epsilon n(\epsilon) \sim k log(\frac{\epsilon}{\epsilon_p}) $.
• We estimate the range of values for the parameters defining the EBL spectrum, $k$ and $\epsilon_p$, such that the correlation of the intrinsic VHE spectrum with redshift is nullified.