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

Abstract

With the arrival of powerful, ground-based γ-ray detectors, we can now begin to seriously probe, via simultaneous X-ray/TeV observations, the origin of the γ-ray emission in the blazars Mrk 421 and 501. If the synchrotron-Compton emission model turns out to work, then we know that the same electrons are responsible for both the X-ray and the γ-ray emission of these objects. In this case, we show that we can use their observed X-ray spectra to robustly estimate their intrinsic γ-ray spectra. Among blazars, Mrk 421/501 are particularly well suited for this task because the Compton scattering which produces their TeV γ-ray is likely to be in the Klein-Nishina limit, where the outgoing photon has an energy insensitive to the incoming photon energy. With a better handle on their intrinsic TeV spectra, we can then begin to search for evidence of absorption due to γ-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 (≳ 20 TeV in Mrk 501) that we may nevertheless be able to derive interesting constraints on the infrared background. If correct, the combination of the COBE 140 μ detection and the measurement of Mrk 501's spectrum out to beyond ∼ 20 TeV rules out conventional galaxy evolution and star formation scenarios, implying that much of the star formation in the Universe indeed occurs at early times in highly obscured sources that have been missed until now.