Probing axion-like particles with the ultraviolet photon polarization from active galactic nuclei in radio galaxies

The mixing of photons with axion-like particles (ALPs) in the large-scale magnetic field $B$ changes the polarization angle of a linearly polarized photon beam from active galactic nuclei in radio galaxies as it propagates over cosmological distances. Using available ultraviolet polarization data concerning these sources we derive a new bound on the product of the photon-ALP coupling $g_{a\gamma}$ times $B$. We find $g_{a\gamma} B \lesssim 10^{-11}$ GeV$^{-1}$ nG for ultralight ALPs with $m_a \lesssim 10^{-15}$ eV. We compare our new bound with the ones present in the literature and we comment about possible improvements with observations of more sources.Comment: v2: one typo corrected. Added a few comments, matches published versio

Very-high-energy quasars hint at ALPs

One of the mysteries of very-high-energy (VHE) astrophysics is the observation of flat spectrum radio quasars (FSRQs) above about 30 GeV, because at those energies their broad line region should prevent photons produced by the central engine to escape. Although a few astrophysical explanations have been put forward, they are totally ad hoc. We show that a natural explanation emerges within the conventional models of FSRQs provided that photon-ALP oscillations take place inside the source for the model parameters within an allowed range.Comment: 4 pages, 3 figures. Proceeding of the workshop "9th Patras Workshop on Axions, WIMPs and WISPs", 24 - 28 June 2013, Schloss Waldhausen, Mainz, Germany (to be published in the Proceedings

Gamma ray emission from a baryonic dark halo

A recent re-analysis of EGRET data by Dixon et al. has led to the discovery of a statistically significant diffuse $\gamma$-ray emission from the galactic halo. We show that this emission can naturally be accounted for within a previously-proposed model for baryonic dark matter, according to which dark clusters of brown dwarfs and cold self-gravitating $H_2$ clouds populate the outer galactic halo and can show up in microlensing observations. Basically, cosmic-ray protons in the galactic halo scatter on the clouds clumped into dark clusters, giving rise to the observed $\gamma$-ray flux. We derive maps for the corresponding intensity distribution, which turn out to be in remarkably good agreement with those obtained by Dixon et al. We also address future prospects to test our predictions.Comment: 22 pages, 2 figures, slightly shortened version. to appear in New Journal of Physic

Detection of distant AGN by MAGIC: the transparency of the Universe to high-energy photons

The recent detection of blazar 3C279 by MAGIC has confirmed previous indications by H.E.S.S. that the Universe is more transparent to very-high-energy gamma rays than previously thought. We show that this fact can be reconciled with standard blazar emission models provided photon oscillations into a veri light Axion-Like Particle occur in extragalactic magnetic fields. A quantitative estimate of this effect explains the observed spectrum of 3C279. Our prediction can be tested in the near future by the satellite-borne GLAST detector as well as by the ground-based Imaging Atmospheric Cherenkov Telescpoes H.E.S.S., MAGIC, CANGAROO III, VERITAS and by the Extensive Air Shower arrays ARGO-YBJ and MILAGRO.Comment: 4 pages, 1 figure, Proceeding of the Conference "4th Patras Workshop on Axions, WIMPs and WISPs

Transparency of the Universe to gamma rays

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 gamma-ray photon in the range 10 GeV < E_0 < 10^13 GeV such that the resulting optical depth tau_gamma(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 tau_gamma(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.Comment: 5 pages, 2 figures, MNRAS (2013) - in Pres