D. Wouters

Irregularity in gamma ray source spectra as a signature of axionlike particles

G. Galanti and M .Roncadelli recently made public some comments on the article by D. Wouters and P. Brun about irregularities induced by photon mixing to axion-like particles in astrophysical media [Phys. Rev. D86, 043005 (2012)]. They claim in particular to have found some mistakes in the article. This note is a response to their comments, we refute their arguments and show that the results presented in the article are correct. It turns out most of the misunderstandings come from the definition of the beam initial state, some clarifications about which are given here.

CONSTRAINTS ON AXION-LIKE PARTICLES FROM X-RAY OBSERVATIONS OF THE HYDRA GALAXY CLUSTER

Axion-like particles (ALPs) belong to a class of new pseudoscalar particles that generically couple to photons, opening the possibility of oscillations from photons into ALPs in an external magnetic field. Having witnessed the turbulence of their magnetic fields, these oscillations are expected to imprint irregularities on a limited energy range of the spectrum of astrophysical sources. In this study, Chandra observations of the Hydra galaxy cluster are used to constrain the value of the coupling of ALPs to photons. We consider the conversion of X-ray photons from the central source Hydra A in the magnetic field of the cluster. The magnetic field strength and structure are well determined observationally, which adds to the robustness of the analysis. The absence of anomalous irregularities in the X-ray spectrum of Hydra A conservatively provides the most competitive constraints on the coupling constant for ALP masses below 7 × 10–12 eV at the level of g γa < 8.3 × 10–12 GeV–1 at the 95% confidence level. Because of the specific phenomenology involved, these constraints actually hold more generally for very light pseudo-Nambu-Goldstone bosons.

Anisotropy test of the axion-like particle Universe opacity effect: a case for the Cherenkov Telescope Array

The opacity of the Universe to gamma rays is still poorly measured, in particular anomalies may have been observed. Assuming that such anomalies find their origin in conventional physics like intrinsic source spectra or the density of the extragalactic background light, they would be evenly distributed over the sky. If they exist, axion-like particles (ALPs) would have a potential effect on the opacity of the Universe to gamma rays, possibly related to the anomalies in the spectral indices of distant gamma-ray sources. Under the assumption that distant sources offer suitable environments for their production, ALPs convert back to photons in the Galactic magnetic field and their effect on the opacity is expected to depend on the position of the sources. In that case the anomaly is expected to exhibit peculiar correlations on the sky. We propose a method to test the origin of the opacity anomaly, based on angular correlations of spectral softening anomalies. Such a diagnosis requires a wide-field survey of high-energy gamma-ray sources over a broad range of energy. The future Cherenkov Telescope Array (CTA) is perfectly suited to perform such a study. It is shown that while the current sample of sources is not large enough to base conclusions on, with this method CTA will be sensitive to ALP couplings to gamma rays of the order of 3 ? 10?11GeV?1 for ALP masses below 10?8 eV.

Galaxy cluster constraints on the coupling to photons of low-mass scalars

We consider a broad class of interactions between radiation and a light scalar field, including both conformal and disformal couplings. Such a scalar field potentially acts on cosmological scales as dark energy and could also appear in modified gravity theories. We study the consequences of these couplings on the mixing between the scalar field and photons in galaxy clusters in the presence of a magnetic field. In particular we focus on the resulting turbulence-induced irregularities in the X-ray and UV bands. We find new bounds on the photon-to-scalar couplings, both conformal and disformal, which complement laboratory experiments and other astrophysical constraints.

H.E.S.S. Observations of the distant BL Lac PKS 0301-243

PKS 0301-243 is a distant (z=0.266) high-frequency-peaked BL Lac object, detected both at high and very high energies (VHE) with Fermi/LAT and H.E.S.S., and which experienced a flare in May 2010 in the GeV band. H.E.S.S. observations of PKS 0301-243 carried out between September 2009 and December 2011 result in a strong detection of VHE γ-rays from the source with a significance of 9.8 standard deviations. Multi-wavelength observations from optical to GeV gamma-rays are also presented. The VHE spectrum is used to derive an upper limit on the opacity of the universe to gamma-rays.