1,721,027 research outputs found
Evidence for a new light spin-zero boson from cosmological gamma-ray propagation?
No full textRecent findings by imaging atmospheric Cherenkov telescopes indicate a large transparency of the Universe to gamma rays, which can be hardly explained within the current models of extragalactic background light. We show that the observed transparency is naturally produced by an oscillation mechanism-which can occur inside intergalactic magnetic fields-whereby a photon can become a new spin-zero boson with mass m < 10(-10) eV. Because the latter particle travels unimpeded throughout the Universe, photons can reach the observer even if the distance from the source considerably exceeds their mean free path. We compute the expected flux of gamma rays from blazar 3C279 at different energies. Our predictions can be tested in the near future by the gamma-ray telescopes H.E.S.S., MAGIC, CANGAROO, and VERITAS. Moreover, our result provides an important observational test for models of dark energy wherein quintessence is coupled to the photon through an effective dimension-five operator
Constraints on large-scale magnetic fields from the Auger results
No full textA recent article from the Pierre Auger Collaboration links the direction of charged cosmic rays to possible extragalactic sites of emission. The correlation of the direction of such particles with the direction of the emitter allows constraining the value of large-scale magnetic fields B. Assuming for B a coherence length lambda in the range between 1 Mpc and 10 Mpc, we find values of B between 0.3 and 0.9 nG
A case for a baryonic dark halo
No full textRecent observations of microlensing events in the Large Magellanic Cloud by the MACHO and EROS collaborations suggest that an important fraction of the galactic halo is in the form of Massive Halo Objects (MHO) with mass . We outline a scenario in which dark clusters of MHO with mass and molecular clouds form in the halo at galactocentric distances larger than 10 - 20 kpc, provided baryons are a major constituent of the halo. Possible signatures of our picture are discussed
Axion-like particles, cosmic magnetic fields and gamma-ray astrophysics
No full textAbstractAxion-like particles (ALPs) are predicted by many extensions of the Standard Model and give rise to characteristic dimming and polarization effects in a light beam travelling in a magnetic field. In this Letter, we demonstrate that photon-ALP mixing in cosmic magnetic fields produces an observable distortion in the energy spectra of distant gamma-ray sources (like AGN) for ranges of the ALP parameters allowed by all available constraints. The resulting effect is expected to show up in the energy band 100 MeV–100 GeV, and so it can be searched with the upcoming GLAST mission
X-ray emission from dark clusters of MACHOs
No full textMACHOs (Massive Astrophysical Compact Halo Objects) - as discovered by microlensing experiments towards the LMC - provide a natural explanation for the galactic halo dark matter. A realistic possibility is that MACHOs are brown dwarfs of mass similar to 0.1 M.. Various arguments suggest that brown dwarfs should have a coronal X-ray emission of similar to 10(27) erg s(-1). As MACHOs are presumably clumped into dark clusters (DCs), each DC is expected to have a total X-ray luminosity of similar to 10(30) - 10(32) erg s(-1). We discuss the observational prospects for DCs in the X-ray band
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