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Flat spectrum radio quasars (FSRQs) are a particular class of blazars rich ofoptical/ultraviolet photons inside the broad line region (BLR), necessarilyimplying a huge optical depth for $\gamma$ rays above 20 GeV. As a consequence,photons with energy above such a threshold should not be emitted. However,photons in the energy range 70 - 400 GeV have been observed by MAGIC from theFSRQ PKS 1222+216. Several astrophysical explanations exist in the literature,but they are all ad hoc, namely devised only for that specific purpose. We showthat such a surprising discovery can be explained within standard blazar modelsby adding the new possibility that photons oscillate into axion-like particles(ALPs) and vice-versa inside the source. Through the photon-ALP oscillationmechanism a sizable fraction of very-high energy (VHE) photons can escapeabsorption from the BLR in a similar fashion as they largely avoid absorptionfrom the extragalactic background light (EBL) in the intergalactic space.Actually, we show that not only are VHE photon indeed emitted, but also thattheir spectral energy distribution (SED) is such that they lie on the sameCompton peak to which also lower energy photons simultaneously detected byFermi/LAT belong.

Hints for an axion-like particle from PKS 1222+216?