Axion Like Particles (ALPs) are predicted to couple with photons in thepresence of magnetic fields. This effect may lead to a significant change inthe observed spectra of gamma-ray sources such as AGNs. Here we carry out adetailed study that for the first time simultaneously considers in the sameframework both the photon/axion mixing that takes place in the gamma-ray sourceand that one expected to occur in the intergalactic magnetic fields. Anefficient photon/axion mixing in the source always means an attenuation in thephoton flux, whereas the mixing in the intergalactic medium may result in adecrement and/or enhancement of the photon flux, depending on the distance ofthe source and the energy considered. Interestingly, we find that decreasingthe value of the intergalactic magnetic field strength, which decreases theprobability for photon/axion mixing, could result in an increase of theexpected photon flux at Earth if the source is far enough. We also find a 30%attenuation in the intensity spectrum of distant sources, which occurs at anenergy that only depends on the properties of the ALPs and the intensity of theintergalactic magnetic field, and thus independent of the AGN source beingobserved. Moreover, we show that this mechanism can easily explain recentpuzzles in the spectra of distant gamma-ray sources... [ABRIDGED] Theconsequences that come from this work are testable with the current generationof gamma-ray instruments, namely Fermi (formerly known as GLAST) and imagingatmospheric Cherenkov telescopes like CANGAROO, HESS, MAGIC and VERITAS.
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