Hard TeV spectra of blazars and the constraints to the IR intergalactic background

Abstract

Recent gamma-ray observations of the blazar 1ES 1101-232 (redshift z=0.186) reveal that the unabsorbed TeV spectrum is hard, with spectral index $\alpha \lesssim 0.5$ [$F(\nu )\propto {\nu }^{-\alpha }$]. We show that simple one-zone synchrotron self-Compton model can explain such hard spectra if we assume a power law energy distribution of the emitting electrons with a relatively high minimum energy. In this case the intrinsic TeV spectrum can be as hard as $F(\nu )\propto {\nu }^{1/3}$, while the predicted X-ray spectrum can still be much softer. The observations of 1ES 1101-232 can therefore be reconciled with relatively high intensities of the infrared background, even if some extreme background levels can indeed be excluded. We show that the other TeV sources (Mrk 421, Mrk 501 and PKS 2155-304) can be interpreted in the same framework, with a somewhat larger minimum energy.

5 pages, 3 figures, accepted for publication as a letter in MNRAS

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