Simultaneous Constraints on the Spectrum of the Extragalactic Background   Light and the Intrinsic Tev Spectra of Mrk 421, Mrk 501, and H1426+428
  Very high energy (~ TeV) $\gamma$-rays from blazars are attenuated by photons from the extragalactic background light (EBL). Observations of blazars can therefore provide an ideal opportunity for determining the EBL intensity if their intrinsic spectrum is known. Conversely, knowledge of the EBL intensity can be used to determine the intrinsic blazar spectrum. Unfortunately, neither the EBL intensity nor the intrinsic blazar spectrum is known with high enough precision to accurately derive one quantity from the other. In this paper we use the most recent data on the EBL to construct twelve different realizations representing all possible permutations between EBL limits and the detections in the different wavelength regions. We use these realizations to explore the effects of the EBL on the inferred spectra of blazars. In particular, we show that the frequently cited "IR background-TeV gamma-ray crisis" does not exist, and derive the intrinsic spectra and peak energies of the blazars Mrk 421, 501 and H1426+428 for EBL realizations that give rise to physically viable intrinsic blazar spectra. We also show that the intrinsic spectrum of Mrk~421 during a period of intense flaring activity has a peak energy that seems to shift to higher energies at higher flux states. Finally, we also explore the effect of the uncertainties in the absolute calibration of the gamma-ray energies on derived TeV opacities and the intrinsic blazar spectra. 
Eli Dwek (NASA Goddard Space Flight Center) & Frank Krennrich (Iowa   State University)