Searching for photonALPs mixing effects in AGN gammaray energy spectra
Abstract
High energy gammarays propagating in external magnetic fields may convert into axionlike particles (ALPs). In this case, the observed gammaray spectra are modified by the resulting energydependent conversion probability. In this study, we use the energy spectra of 20 extragalactic gammaray sources recorded during 10 years of FermiLAT observations. We define a test statistics based upon the likelihood ratio to test the hypothesis for a spectral model without vs. a model with photonALPs coupling. The conversion probability is calculated for fixed values of the mass and twophoton coupling of the pseudoscalar particle while the external magnetic field is characterized by the additional free parameters length scale s and average field strength B. As a consistency check and in order to extend the analysis to include very high energy gammaray data, another test statistics is defined with the χ^{2} method. We find for 18 of the 20 sources a favorable fit, particularly for Markarian 421 and NGC 1275 a significant improvement, with the hypothesis of photonALPs coupling in likelihood analysis. The test statistics of the sources are combined and the significance has been estimated 5.3σ (test statistics summed in local maxima of all sources) and 6.0σ (global maxima). The significance is estimated from dedicated simulations under the null hypotheses. The locally bestfitting values of B and s fall into the range that is expected for large scale magnetic fields present in relevant astrophysical environments.
 Publication:

Journal of Cosmology and Astroparticle Physics
 Pub Date:
 May 2023
 DOI:
 10.1088/14757516/2023/05/029
 arXiv:
 arXiv:2208.00079
 Bibcode:
 2023JCAP...05..029Y
 Keywords:

 gamma ray experiments;
 magnetic fields;
 Statistical sampling techniques;
 Astrophysics  High Energy Astrophysical Phenomena
 EPrint:
 40pages, 45figures