1 2009 Fermi Symposium, Washington, D.C., Nov. 2-5 arXiv:1001.1892v1 [astro-ph.CO] 12 Jan 2010 Miguel A. S´ anchez-Conde Instituto de Astrof´ısica de Canarias, E-38205 La Laguna, Tenerife, Spain David Paneque and Elliott Bloom Kavli Institute for Particle Astrophysics and Cosmology (KIPAC), SLAC National Accelerator Center, Sand Hill Road 2575, CA 94025, USA The search of axion-like-particles with Fermi and Cherenkov telescopes ∗ Francisco Prada Instituto de Astrof´ısica de Andaluc´ıa (CSIC), E-18008, Granada, Spain Alberto Dom´ınguez Instituto de Astrof´ısica de Andaluc´ıa (CSIC), E-18008, Granada, Spain and Departamento de F´ısica At´ omica, Molecular y Nuclear, Universidad de Sevilla, E-41012, Sevilla, Spain Axion Like Particles (ALPs), postulated to solve the strong-CP problem, are predicted to couple with photons in the presence of magnetic fields, which may lead to a significant change in the observed spectra of gamma-ray sources such as AGNs. Here we simultaneously consider in the same framework both the photon/axion mixing that takes place in the gamma-ray source and that one expected to occur in the intergalactic magnetic fields. We show that photon/axion mixing could explain recent puzzles regarding the observed spectra of distant gamma-ray sources as well as the recently published lower limit to the EBL intensity. We finally summarize the different signatures expected and discuss the best strategy to search for ALPs with the Fermi satellite and current Cherenkov telescopes like CANGAROO, HESS, MAGIC and VERITAS. in the range 0.02-300 GeV. Given a domain of length s , where there is a roughly constant magnetic field and plasma frequency, the probability of a photon of energy E$_{γ}$ to be converted into an ALP after traveling through it can be written as [9, 11]: Here Δ$_{osc}$ is the oscillation wave number: Δ$_{B}$ that gives us an idea of how effective is the mixing, i.e. Axions were postulated to solve the strong-CP problem in QCD in the 1970s [2], and are still the most compelling explanation for it. Moreover, they are also appealing for astrophysicists, since they are valid dark matter candidates to constitute a portion or the totality of the non-barionic cold dark matter con- tent predicted to exist in the Universe. But probably the most interesting property of axions, or in a more generic way, Axion-Like Particles (ALPs), where the mass m$_{a}$ and the coupling constant are not related to each other, is that they are expected to oscillate into photons (and viceversa) in the presence of magnetic fields [3, 4]. This oscillation of photons to ALPs are the main vehicle used at present in axion searches, like those carried out by CAST [5] or ADMX [6], but they could also have important implications for astronom- ical observations. For example, they could distort the spectra of gamma-ray sources, such as Active Galactic Nuclei (AGNs) [7–10] or galactic sources in the TeV range [11]. These distortions may be detected by cur- rent gamma-ray experiments, such as Imaging Atmo- spheric Cherenkov Telescopes (IACTs) like MAGIC [12], HESS [13], VERITAS [14] or CANGAROO-III [15], covering energies in the range 0.1-20 TeV, and by the Fermi satellite [16], which operates at energies where B$_{t}$ the magnetic field component along the po- larization vector of the photon and M 11 the inverse of the coupling constant. Δ$_{CM}$ is the vacuum Cotton-Mouton term, i.e. where B$_{cr}$ = m 2 $_{e}$/e ≃ 4 . 41 × 10 13 G the critical mag- netic field strength ( e is the electron charge). ∗ A summary of the work presented in Ref.[1] eConf C091122 Δ$_{CM}$ = − α 45 π ( B$_{t}$ B$_{cr}$ ) 2 E$_{γ}$ ≃ − 1 . 3 × 10 − 21 B 2 mG ( E$_{γ}$ TeV ) cm − $^{1}$, (4) Δ$_{B}$ = B$_{t}$ 2 M ≃ 1 . 7 × 10 − 21 M 11 B$_{mG}$ cm − $^{1}$, (3) Δ 2 osc ≃ (Δ$_{CM}$ + Δ$_{pl}$ − Δ$_{a}$) 2 + 4Δ 2 $_{B}$, (2) P 0 = (Δ$_{B}$ s ) 2 sin $^{2}$(Δ$_{osc}$ s/ 2) (Δ$_{osc}$ s/ 2) 2 (1) I. PHOTON/AXION OSCILLATIONS