5 2009 Fermi Symposium, Washington, D.C., Nov. 2-5 FIG. 4: Summary of the best observational strategy to look for ALPs with Fermi and IACTs. See Ref. [1] for a deeper discussion. e.g.: If we accurately knew the intrinsic spectrum of the sources and/or the density of the EBL, we should be able to observationally detect axion signatures or to exclude some portions of the parameter space. We lack this knowledge, so detection is challenging but we believe that still possible. The combination of the Fermi/LAT instrument and the IACTs, which cover 6 decades in energy (from 20 MeV to 20 TeV) is very well suited to study the photon/axion mixing effect. Nevertheless, and before assuming an scenario with axions to interpret the observational data, we should try to describe the observational data (preferably sev- eral AGNs located at different redshifts, as well as the same AGN undergoing different flaring states, from radio to TeV) with “conventional” theoretical models for the AGN emission and for the EBL. If these “con- ventional” models for the source emission and EBL fail (i.e. if we have important residuals for the best-fit model), then the axion scenario should be explored. Fig. 4 summarizes what could be a good observational strategy to look for ALPs with Fermi and IACTs. While it is still possible to explain these observa- tions with conventional physics, the axion/photon os- cillation would naturally explain these puzzles, since we get more high energy photons than expected as well as a softer intrinsic spectrum in a scenario with ALPs. An example is given in Fig. 5 (from Ref. [27]), where the effect of the existence of ALPs in the intrin- sic spectrum of 3C 279 is shown assuming the fiducial model presented in Table I and a Kneiske best-fit EBL model. III. DETECTION PROSPECTS FOR FERMI AND IACTS • The VERITAS Collaboration recently claimed a detection above 0.1 TeV coming from 3C66A (z=0.444). The EBL-corrected spectrum seems to be harder than 1.5 [21]. • TeV photons coming from 3C 66A? (see Refs. [22–24]). If so, difficult to explain with conven- tional EBL models and physics. • The lower limit on the EBL at 3.6 µ m was re- cently revised upwards by a factor ∼ 2, suggest- ing a more opaque universe [25]. • Some sources at z = 0.1-0.2 seem to have harder intrinsic energy spectra than previously antici- pated [26]. V. CONCLUSIONS IV. ARE WE DETECTING AXIONS ALREADY? If axions exist, then they could distort the spec- tra of astrophysical sources importantly. In addition, Recent gamma observations might already pose sub- stantial challenges to the conventional models to ex- plain the observed source spectra and/or EBL density, eConf C091122