Hardening of TeV gamma spectrum of active galactic nuclei in galaxy clusters by conversions of photons into axionlike particles

Abstract

A fraction of active galactic nuclei producing very high-energy (VHE) rays are located in galaxy clusters. The magnetic field present in the intracluster medium would lead to conversions of VHE photons into axionlike particles (ALPs), which are a generic prediction of several extensions of the Standard Model. ALPs produced in this way would traverse cosmological distances unaffected by the extragalactic background light at variance with VHE photons which undergo a substantial absorption. Eventually, a nontrivial fraction of ALPs would reconvert into VHE photons in the magnetic field of the Milky Way. This mechanism produces a significant hardening of the VHE spectrum of active galactic nuclei in galaxy clusters. As a specific example we consider the energy spectra of two observed VHE -ray sources located in galaxy clusters, namely 1ES 0414 þ 009 at redshift z ¼ 0:287 and Mkn 501 at z ¼ 0:034. We find that the hardening in the observed spectra becomes relevant at E * 1 TeV.

Simulations of extragalactic magnetic fields and of their observables

“…Only a fraction of them is finally transformed back into photons near the observers. The first study of ALP oscillations used analytical prescriptions for the magnetic field distributions in galaxy clusters and voids (Horns et al, 2012). In recent work based on the primordial 2400 3 run of the Chronos++ suite, we simulated the propagation of photons from redshift z = 1 and computed the expected conversion into ALPs (Montanino et al, 2017).…”

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); .…”

Axion-like particles and the propagation of gamma rays over astronomical distances

“…Alternatively, it may happen in the galaxy clusters or filaments surrounding the source and/or the observer [24] (see also a more detailed subsequent study in Ref. [29]). In this case, a part of emitted photons is converted to ALPs and then travel intact to the vicinity of the observer, while remaining photons attenuate in a usual way.…”