Alessandro De Angelis

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.

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.

Importance of axion-like particles for very-high-energy astrophysics

Several extensions ol the Standard Model predict the existence ol Axion-Like Particles (ALPs), very light spin-zero bosons with a two-photon coupling. ALPs can give rise to observable effects in very-high-energy astrophysics. Above roughly 100 GeV the horizon of the observable Universe progressively shrinks as the energy increases, due to scattering of beam photons off background photons in the optical and infrared bands, which produces e+ e− pairs. In the presence of large-scale magnetic fields photons emitted by a blazar can oscillate into ALPs on the way to us and back into photons before reaching the Earth. Since ALPs do not interact with background photons, the effective mean free path of beam photons increases, enhancing the photon survival probability. While the absorption probability increases with energy, photon-ALP oscillations are energy-independent, and so the survival probability increases with energy compared to standard expectations. We have performed a systematic analysis of this effect, interpreting the present data on very-high-energy photons from blazars. Our predictions can be tested with presently operating Cherenkov Telescopes like H.E.S.S., MAGIC, VERITAS and CANGAROO III as well as with detectors like ARGO-YBJ and MILAGRO and with the planned Cherenkov Telescope Array and the HAWC γ-ray observatory. ALPs with the right properties to produce the above effects can possibly be discovered by the GammeV experiment at FERMILAB and surely by the planned photon regeneration experiment ALPS at DESY.

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.

Glast, the gamma-ray large area space telescope

GLAST, a detector for cosmic gamma rays in the range from 20 MeV to 300 GeV, will be launched in space in 2005. Breakthroughs are expected in particular in the study of particle acceleration mechanisms in space and of gamma ray bursts, and maybe on the search for cold dark matter; but of course the most exciting discoveries could come from the unexpected.

STUDY OF INTERMITTENCY IN HADRONIC Z0 DECAYS

The study of the factorial moments of the rapidity distribution and of the (y, ϕ) distribution for hadrons produced in the decay of the Z0, by means of the DELPHI detector at LEP, has shown no deviations from the predictions of Parton-Shower models. Intermittent behavior observed in the latter case has been found to be compatible with jet cascading mechanism.

Atmospheric ionization and cosmic rays: studies and measurements before 1912

Abstract The discovery of cosmic rays, a milestone in science, was based on the work by scientists in Europe and the New World and took place during a period characterized by nationalism and lack of communication. Many scientists that took part in this research a century ago were intrigued by the penetrating radiation and tried to understand the origin of it. Several important contributions to the discovery of the origin of cosmic rays have been forgotten; historical, political and personal facts might have contributed to their substantial disappearance from the history of science.

Intergalactic absorption and blazar gamma-ray spectra

The distribution of TeV spectral slopes versus redshift for currently known TeV blazars (16 sources with z ≤ 0.21, and one with z > 0.25) is essentially a scatter plot with hardly any hint of a global trend. We suggest that this is the outcome of two combined effects of intergalactic γγ absorption, plus an inherent feature of the SSC (synchro-self-Compton) process of blazar emission. First, flux dimming introduces a bias that favors detection of progressively more flaring sources at higher redshifts. According to mainstream SSC models, more flaring source states imply sources with flatter TeV slopes. This results in a structured relation between intrinsic TeV slope and redshift. The second effect, spectral steepening by intergalactic absorption, affects sources progressively with distance and effectively wipes out the intrinsic slope-redshift correlation.

Improved limits on photon velocity oscillations

The mixing of the photon with a hypothetical sterile paraphotonic state would have consequences on the cosmological propagation of photons. The absence of distortions in the optical spectrum of distant Type Ia supernovae allows to extend by two orders of magnitude the previous limit on the Lorentz-violating parameter δ associated to the photon–paraphoton transition, extracted from the absence of distortions in the spectrum of the cosmic microwave background. The new limit is consistent with the interpretation of the dimming of distant Type Ia supernovae as a consequence of a nonzero cosmological constant. Observations of gamma-rays from active galactic nuclei allow to further extend the limit on δ.

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.