Understanding the spectra of TeV blazars: implications for the cosmic infrared background

With the arrival of powerful, ground-based gamma-ray detectors, we can now begin to seriously probe, via simultaneous X-ray/TeV observations, the orig in of the gamma-ray emission in the blazars Mrk 421 and 501. If the synchro tron-Compton emission model turns out to work, then we know that the same e lectrons are responsible for both the X-ray and the gamma-ray emission of t hese objects. In this case, we show that we can use their observed X-ray sp ectra to robustly estimate their intrinsic gamma-ray spectra, Among blazars , Mrk 421/501 are particularly well suited for this task because the Compto n scattering which produces their TeV gamma-rays is likely to be in the Kle in-Nishina limit, where the outgoing photon has an energy insensitive to th e incoming photon energy. With a better handle on their intrinsic TeV spect ra, we can then begin to search for evidence of absorption due to gamma-ray pair production on diffuse infrared background radiation. We discuss some of the pitfalls that arise when one attempts to do this without knowing the intrinsic spectrum. Even though Mrk 421/501 are very nearby, the emission of these sources extends to sufficiently high energies (greater than or sim ilar to 20 TeV in Mrk 501) that we may nevertheless be able to derive inter esting constraints on the infrared background. If correct, the combination of the COBE 140 mu detection acid the measurement of Mrk 501's spectrum out to beyond similar to 20 TeV rules out conventional galaxy evolution and st ar formation scenarios, implying that much of the star formation in the Uni verse indeed occurs at early times in highly obscured sources that have bee n missed until now. (C) 1999 Elsevier Science B.V. All rights reserved.