O. Mansutti

Evidence for a new light spin-zero boson from cosmological gamma-ray propagation?

Recent 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{sup -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.

Photon propagation and the very high energy γ-ray spectra of blazars: how transparent is the Universe?

Recent findings by γ -ray Cherenkov telescopes suggest a higher transparency of the Universe to very high energy (VHE) photons than expected from current models of the extragalactic background light. It has been shown that such transparency can be naturally explained by the DARMA scenario, in which the photon mixes with a new, very light, axion-like particle predicted by many extensions of the Standard Model of elementary particles. We discuss the implications of DARMA for observations of blazar VHE γ -ray spectra, and show that it successfully accounts for the observed correlation between spectral slope and redshift when the same intrinsic emission spectrum is adopted for faraway sources and for nearby ones. DARMA also predicts the observed blazar spectral index to become asymptotically independent of redshift for faraway sources. Our prediction can be tested with the satellite-borne Fermi/LAT (Large Area Telescope) detector as well as with the ground-based Cherenkov telescopes HESS, MAGIC, CANGAROO III, VERITAS and the Extensive Air Shower arrays ARGO-YBJ and Milagro.

Axion-Like Particles, Cosmic Magnetic Fields and Gamma-Ray Astrophysics

Abstract Axion-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.

Very-high-energy gamma astrophysics

Summary. — High-energy photons are a powerful probe for astrophysics and for fundamental physics under extreme conditions. During the recent years, our knowledge of the most violent phenomena in the Universe has impressively progressed thanks to the advent of new detectors for high-energy γ-rays. Observation of γrays gives an exciting view of the high-energy universe thanks to the satellite-based telescopes (AGILE, GLAST) and to the ground-based detectors like the Cherenkov telescopes (H.E.S.S. and MAGIC in particular), which recently discovered more than 60 new very-high-energy sources. The progress achieved with the last generation of Cherenkov telescopes is comparable to the one drawn by EGRET with respect to the previous γ-ray satellite detectors. This paper reviews the present status of high-energy gamma astrophysics, with emphasis on the recent results and on the experimental developments.

Evidence for a new light boson from cosmological gamma-ray propagation?

An anomalously large transparency of the Universe to gamma rays has recently been discovered by the Imaging Atmospheric Cherenkov Telescopes (IACTs) H.E.S.S. and MAGIC. We show that observations can be reconciled with standard blazar emission models provided photon oscillations to a very light Axion‐Like Particle occur in extragalactic magnetic fields. A quantitative estimate of this effect is successfully applied to blazar 3C279. Our predictions can be tested in the near future by new observations performed both with IACTs and with the ARGO‐YBJ air shower detector. Our result also offers an important observational test for models of dark energy wherein quintessence is coupled to the photon through an effective dimension‐five operator.

THE MAGIC EXPERIMENT AND ITS FIRST RESULTS

With its diameter of 17m, the MAGIC telescope is the largest Cherenkov detector for gamma ray astrophysics. It is sensitive to photons above an energy of 30 GeV. MAGIC started operations in October 2003 and is currently taking data. This report summarizes its main characteristics, its first results and its potential for physics.

A new light boson from MAGIC observations

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 currently thought. This circumstance can be reconciled with observations of nearby blazars provided that photon oscillations into a very light Axion-Like Particle occur in extragalactic magnetic fields. The emerging “DARMA scenario” can be tested in the near future by the satellite-borne Fermi LAT detector as well as by the ground-based Imaging Atmospheric Cherenkov Telescopes H.E.S.S., MAGIC, CANGAROO III, VERITAS and by the Extensive Air Shower arrays ARGO-YBJ and MILAGRO.

A novel background reduction strategy for high level triggers and processing in gamma‐ray Cherenkov detectors

Gamma ray astronomy is now at the leading edge for studies related both to fundamental physics and astrophysics. The sensitivity of gamma detectors is limited by the huge amount of background, constituted by hadronic cosmic rays (typically two to three orders of magnitude more than the signal) and by the accidental background in the detectors. By using the information on the temporal evolution of the Cherenkov light, the background can be reduced. We will present here the results obtained within the MAGIC experiment using a new technique for the reduction of the background. Particle showers produced by gamma rays show a different temporal distribution with respect to showers produced by hadrons; the background due to accidental counts shows no dependence on time. Such novel strategy can increase the sensitivity of present instruments.

Have Cherenkov telescopes detected a new light boson

Recent observations by H.E.S.S. and MAGIC strongly suggest 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 that photon oscillations into a very light Axion-Like Particle occur in extragalactic magnetic fields. A quantitative estimate of this effect indeed explains the observed data and in particular the spectrum of blazar 3C279.

Detecting new very light bosons by Cherenkov telescopes

A generic prediction of several extensions of the Standard Model of elementary‐particle interactions is the existence of axion‐like particles (ALPs), namely very light spin‐zero bosons characterized by a two‐photon coupling. While elusive in laboratory experiments, ALPs can give rise to observable astrophysical effects for their relevant parameters in experimentally allowed ranges. We show that the unexpectedly low opacity of the Universe inferred by the Imaging Atmospheric Cherenkov Telescopes since 2006 from blazar observations above 100 GeV can be explained naturally within the De Angelis, Roncadelli & Mansutti—hereafter DARMA—scenario, namely in terms of photon‐ALP oscillations occurring in extragalactic magnetic fields. We work out the implications of the DARMA scenario for the VHE gamma‐ray spectra of blazars by contemplating all of them at once, so that the emitted Γem and observed Γobs spectral indices can be correlated. We demonstrate that by assuming the same nominal value Γem≃2.4 for all VHE bl...