Giorgio Galanti

Evidence for an axion-like particle from PKS 1222+216?

The surprising discovery by MAGIC of an intense, rapidly varying emission in the energy range 70 - 400 GeV from the flat spectrum radio quasar PKS 1222+216 represents a challenge for all interpretative scenarios. Indeed, in order to avoid absorption of \gamma rays in the dense ultraviolet radiation field of the broad line region (BLR), one is forced to invoke some unconventional astrophysical picture, like for instance the existence of a very compact (r\sim 10^{14} cm) emitting blob at a large distance (R \sim10^{18} cm) from the jet base. We offer the investigation of a scenario based on the standard blazar model for PKS 1222+216 where \gamma rays are produced close to the central engine, but we add the new assumption that inside the source photons can oscillate into axion-like particles (ALPs), which are a generic prediction of several extensions of the Standard Model of elementary particle interactions. As a result, a considerable fraction of very-high-energy photons can escape absorption from the BLR through the mechanism of photon-ALP oscillations much in the same way as they largely avoid absorption from extragalactic background light when propagating over cosmic distances in the presence of large-scale magnetic fields in the nG range. In addition we show that the above MAGIC observations and the simultaneous Fermi/LAT observations in the energy range 0.3 - 3 GeV can both be explained by a standard spectral energy distribution for experimentally allowed values of the model parameters. In particular, we need a very light ALP just like in the case of photon-ALP oscillations in cosmic space. Moreover, we find it quite tantalizing that the most favorable value of the photon-ALP coupling happens to be the same in both situations. Although our ALPs cannot contribute to the cold dark matter, they are a viable candidate for the quintessential dark energy. [abridged]

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.

Photons to axion-like particles conversion in Active Galactic Nuclei

a b s t r a c t The idea that photons can convert to axion-like particles (ALPs) γ → a in or around an AGN and reconvert back to photons a → γ in the Milky Way magnetic field has been put forward in 2008 and has recently attracted growing interest. Yet, so far nobody has estimated the conversion probability γ → a as carefully as allowed by present-day knowledge. Our aim is to fill this gap. We first remark that AGN which can be detected above 100 GeV are blazars, namely AGN with jets, with one of them pointing towards us. Moreover, blazars fall into two well defined classes: BL Lac objects (BL Lacs) and Flat Spectrum Radio Quasars (FSRQs), with drastically different properties. In this Letter we report a preliminary evaluation of the γ → a conversion probability inside these two classes of blazars. Our findings are surprising. Indeed, while in the case of BL Lacs the conversion probability turns out to be totally unpredictable due to the strong dependence on the values of the somewhat uncertain position of the emission region along the jet and strength of the magnetic field therein, for FSRQs we are able to make a clear-cut prediction. Our results are of paramount importance in view of the planned very-high-energy photon detectors like the CTA, HAWK, GAMMA-400 and HISCORE.

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.

Hints for an axion-like particle from PKS 1222+216?

Flat spectrum radio quasars (FSRQs) are a particular class of blazars rich of optical/ultraviolet photons inside the broad line region (BLR), necessarily implying a huge optical depth for

Extragalactic photon–axion-like particle oscillations up to 1000 TeV

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Advantages of axion-like particles for the description of very-high-energy blazar spectra

rays above 20 GeV. As a consequence, photons with energy above such a threshold should not be emitted. However, photons in the energy range 70 - 400 GeV have been observed by MAGIC from the FSRQ PKS 1222+216. Several astrophysical explanations exist in the literature, but they are all ad hoc, namely devised only for that specific purpose. We show that such a surprising discovery can be explained within standard blazar models by adding the new possibility that photons oscillate into axion-like particles (ALPs) and vice-versa inside the source. Through the photon-ALP oscillation mechanism a sizable fraction of very-high energy (VHE) photons can escape absorption from the BLR in a similar fashion as they largely avoid absorption from the extragalactic background light (EBL) in the intergalactic space. Actually, we show that not only are VHE photon indeed emitted, but also that their spectral energy distribution (SED) is such that they lie on the same Compton peak to which also lower energy photons simultaneously detected by Fermi/LAT belong.

Comment on "Irregularity in gamma ray source spectra as a signature of axion-like particles"

Axion-like particles (ALPs) are attracting increasing interest since, among other things, they are a prediction of many extensions of the standard model of elementary particles physics and in particular of superstrings and superbranes. Remarkably, depending on the set of their parameter space, they strongly increase the photon transparency in the very-high energy band. The recent discovery of photon dispersion on the CMB requires a substantial modification of the previous picture: this is indeed the goal of the present paper. We compute the photon survival probability from a blazar to us exactly, and we plot it versus the observed energy for 7 simulated blazars at different

No axion-like particles from core-collapse supernovae?

z

Erratum: Relevance of axionlike particles for very-high-energy astrophysics [Phys. Rev. D84, 105030 (2011)]

and 4 values of a model parameter. Our predictions can be tested by the new generation of