First lower limits on the photon-axion-like particle coupling from very high energy gamma-ray observations

Abstract

The intrinsic flux of very high-energy (VHE, energy 100 GeV) γ-rays from extragalactic sources is attenuated due to pair production in the interaction with photons of the extragalactic background light (EBL). Depending on the distance of the source, the Universe should be opaque to VHE photons above a certain energy. However, indications exist that the Universe is more transparent than previously thought. A recent statistical analysis of a large sample of VHE spectra shows that the correction for absorption with current EBL models is too strong for the data points with the highest attenuation. An explanation might be the oscillation of VHE photons into hypothetical axionlike particles (ALPs) in ambient magnetic fields. This mechanism would decrease the opacity, as ALPs propagate unimpeded over cosmological distances.Here, a large sample of VHE γ-ray spectra obtained with imaging air Cherenkov telescopes is used to set, for the first time, lower limits on the photon-ALP coupling constant gaγ over a large range of ALP masses. The conversion in different magnetic field configurations, including intracluster and intergalactic magnetic fields together with the magnetic field of the Milky Way, is investigated taking into account the energy dependence of the oscillations. For optimistic scenarios of the intervening magnetic fields, a lower limit on gaγ of the order of 10 −12 GeV −1 is obtained, whereas more conservative model assumptions result in gaγ 2 × 10 −11 GeV −1 . The latter value is within reach of future dedicated ALP searches.

Photon-ALP conversion in intergalactic space

“…A number of differences in the analysis exist (combination of Fermi-LAT and ground based measurements, shifting of energy bins, different statistical tests). The interpretation of this pair production anomaly in the context of the appearance channel from photon-ALPs mixing is consistent with a mass of m a < µeV and coupling g aγγ > 10 −11 GeV −1 , not violating any existing bounds [17]. The uncertainty on coupling and mass is a direct result of our ignorance on the morphology and strength of the intervening magnetic field.…”

“…The estimates derived [17] are therefore very uncertain. In the disappearance channel on the other hand, it is possible to reconstruct the parameters by fitting the spectral modification factors to the observed spectra, only a degeneracy with the assumed magnetic field remains.…”

A new bound on axion-like particles

“…Our constraint is also interesting in relation to the suggested ALP explanation of the apparent anomalous transparency of the universe for very high energy cosmic rays [19,[60][61][62][63], which postulate the existence of a light ALP with m a 10 −9 eV and g aγ 3 × 10 −12 GeV −1 . Our bound constrains, but does not rule out, other possible hints of light ALPs such as the soft X-ray excess from galaxy cluster which may be explained by ALP-photon conversion of a relativistic background of ALPs [14,15,17,[64][65][66], and the explanation of the possible unidentified 3.5 keV line from galaxy clusters [67,68], which may be explained by dark matter decaying into ALPs, which subsequently convert to photons [18,69,70].…”

Constraints on Axion-like Particles from X-Ray Observations of NGC1275

“…Following the original papers Sikivie (1983Sikivie ( , 1985; Raffelt & Stodolsky (1988), the physics of ALPs passing through magnetic fields has been described in many works. An incomplete list of articles studying aspects of photon-ALP interconversion in astrophysical magnetic fields includes Brockway et al (1996b); Grifols et al (1996b); Fairbairn et al (2011); Burrage et al (2009) ;Mirizzi & Montanino (2009) ;Tavecchio et al (2012); Payez et al (2012); Horns et al (2012); Meyer et al (2013); Wouters & Brun (2013); Conlon & Marsh (2013); Angus et al (2014); Payez et al (2015); Powell (2015); Dobrynina et al (2015); Schlederer & Sigl (2016); .…”