Hints of an axion-like particle mixing in the GeV gamma-ray blazar data?

摘要

Axion-Like Particles (ALPs), if exist in nature, are expected to mix withphotons in the presence of an external magnetic field. The energy range ofphotons which undergo strong mixing with ALPs depends on the ALP mass, on itscoupling with photons as well as on the external magnetic field and particledensity configurations. Recent observations of blazars by the Fermi Gamma-RaySpace Telescope in the 0.1-300 GeV energy range show a break in their spectrain the 1-10 GeV range. We have modeled this spectral feature for theflat-spectrum radio quasar 3C454.3 during its November 2010 outburst, assumingthat a significant fraction of the gamma rays convert to ALPs in the largescale jet of this blazar. Using theoretically motivated models for the magneticfield and particle density configurations in the kiloparsec scale jet, outsidethe broad-line region, we find an ALP mass $m_a\sim (1-3)\cdot 10^{-7}$ eV andcoupling $g_{a\gamma} \sim (1-3)\cdot 10^{-10}$ GeV$^{-1}$ after performing anillustrative statistical analysis of spectral data in four different epochs ofemission. The precise values of $m_a$ and $g_{a\gamma}$ depend weakly on theassumed particle density configuration and are consistent with the currentexperimental bounds on these quantities. We apply this method and ALPparameters found from fitting 3C454.3 data to another flat-spectrum radioquasar PKS1222+216 (4C+21.35) data up to 400 GeV, as a consistency check, andfound good fit. We find that the ALP-photon mixing effect on the GeV spectramay not be washed out for any reasonable estimate of the magnetic field in theintergalactic media.