We review the physics potential of a next generation search for solar axions: the International Axion Observatory (IAXO). Endowed with a sensitivity to discover axion-like particles (ALPs) with a coupling to photons as small as g aγ ∼ 10 −12 GeV −1 , or to electrons g ae ∼10 −13 , IAXO has the potential to find the QCD axion in the 1 meV∼1 eV mass range where it solves the strong CP problem, can account for the cold dark matter of the Universe and be responsible for the anomalous cooling observed in a number of stellar systems. At the same time, IAXO will have enough sensitivity to detect lower mass axions invoked to explain: 1) the origin of the anomalous "transparency" of the Universe to gamma-rays, 2) the observed soft X-ray excess from galaxy clusters or 3) some inflationary models. In addition, we review string theory axions with parameters accessible by IAXO and discuss their potential role in cosmology as Dark Matter and Dark Radiation as well as their connections to the above mentioned conundrums.

We present an incremental version (4FGL-DR3, for Data Release 3) of the fourth Fermi Large Area Telescope (LAT) catalog of γ-ray sources. Based on the first 12 years of science data in the energy range from 50 MeV to 1 TeV, it contains 6658 sources. The analysis improves on that used for the 4FGL catalog over eight years of data: more sources are fit with curved spectra, we introduce a more robust spectral parameterization for pulsars, and we extend the spectral points to 1 TeV. The spectral parameters, spectral energy distributions, and associations are updated for all sources. Light curves are rebuilt for all sources with 1 yr intervals (not 2 month intervals). Among the 5064 original 4FGL sources, 16 were deleted, 112 are formally below the detection threshold over 12 yr (but are kept in the list), while 74 are newly associated, 10 have an improved association, and seven associations were withdrawn. Pulsars are split explicitly between young and millisecond pulsars. Pulsars and binaries newly detected in LAT sources, as well as more than 100 newly classified blazars, are reported. We add three extended sources and 1607 new point sources, mostly just above the detection threshold, among which eight are considered identified, and 699 have a plausible counterpart at other wavelengths. We discuss the degree-scale residuals to the global sky model and clusters of soft unassociated point sources close to the Galactic plane, which are possibly related to limitations of the interstellar emission model and missing extended sources.

Several extensions of the Standard Model and in particular superstring theories suggest the existence of axion-like particles (ALPs), which are very light spin-zero bosons with a two-photon coupling. As a consequence, photon-ALP oscillations occur in the presence of an external magnetic field, and ALPs can lead to observable effects on the measured photon spectrum of astrophysical sources. An intriguing situation arises when blazars are observed in the very-high-energy (VHE) bandnamely above 100 GeV -as it is the case with the presently operating Imaging Atmospheric Cherenkov Telescopes (IACTs) H.E.S.S., MAGIC, CANGAROO III and VERITAS. The extragalactic background light (EBL) produced by galaxies during cosmic evolution gives rise to a source dimming which becomes important in the VHE band and increases with energy, since hard photons from a blazar scatter off soft EBL photons thereby disappearing into e + e − pairs. This dimming can be considerably reduced by photon-ALP oscillations, and since they are energy-independent the resulting blazar spectra become harder than expected. We consider throughout a scenario first proposed by De Angelis, Roncadelli and Mansutti -to be referred to as DARMA for short -in which the above strategy is implemented with photon-ALP oscillations triggered by large-scale magnetic fields, and we systematically investigate its implications for VHE blazars. We find that for ALPs lighter than 5 · 10 − 10 eV the photon survival probability is larger than predicted by conventional physics above a few hundred GeV. Specifically, a boost factor of 10 can easily occur for sources at large distance and large energy, e.g. at 8 TeV for the blazar 1ES 0347-121 at redshift z = 0.188. This is a clear-cut prediction which can be tested with the planned Cherenkov Telescope Array and the HAWC water Cherenkov γ-ray observatory, and possibly with the currently operating IACTs as well as with detectors like ARGO-YBJ and MILAGRO. Moreover, we show that the DARMA scenario offers a new interpretation of the VHE blazars detected so far, according to which the large spread in the values of the observed spectral index is mainly due to the wide spread in the source distances rather than to large variations of their internal physical properties. Finally, we stress that ALPs with the right properties to produce the above effects can be discovered by the GammeV experiment at FERMILAB and more likely with arXiv:1106.1132v5 [astro-ph.HE] 22 May 2013 2 the planned photon regeneration experiment ALPS at DESY. PACS numbers: 14.80.Mz, 95.85.Pw, 95.85.Ry, 98.70.Rz, 98.70.Vc, 98.70.Sa

Using the most recent observational data concerning the Extragalactic Background Light and the Radio Background, for a source at a redshift z s ≤ 3 we compute the energy E 0 of an observed γ-ray photon in the range 10 GeV ≤ E 0 ≤ 10 13 GeV such that the resulting optical depth τ γ (E 0 , z s ) takes the values 1, 2, 3 and 4.6, corresponding to an observed flux dimming of e −1 0.37, e −2 0.14, e −3 0.05 and e −4.6 0.01, respectively. Below a source distance D 8 kpc we find that τ γ (E 0 , DH 0 /c) < 1 for any value of E 0 . In the limiting case of a local Universe (z s 0) we compare our result with the one derived in 1997 by Coppi and Aharonian. The present achievement is of paramount relevance for the planned ground-based detectors like CTA, HAWC and HiSCORE.

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