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We investigate the axion like particle (ALP)-photon oscillation effect in the high energy $gamma$-ray spectra of PG 1553+113 and PKS 2155-304 measured by Fermi-LAT and H.E.S.S.. The choice of extragalactic background light (EBL) model, which induces the attenuate effect in observed $gamma$-ray spectra, would affect the ALP implication. For the ordinary EBL model that prefers a null hypothesis, we set constraint on the ALP-photon coupling constant at 95% C.L. as $g_{agamma}lesssim 5times 10^{-11} ~rm{GeV}^{-1}$ for the ALP mass $sim 10$ neV. We also consider the CIBER observation of the cosmic infrared radiation, which shows an excess at the wave wavelength of $sim 1~mu$m after the substraction of foregrounds. The high energy gamma-rays from extragalactic sources at high redshifts would suffer from a more significant attenuate effect caused by this excess. In this case, we find that the ALP-photon oscillation would improve the fit to the observed spectra of PKS 2155-304 and PG 1553+113 and find a favored parameter region at 95% C.L..
The addition of a 28 m Cherenkov telescope (CT5) to the H.E.S.S. array extended the experiments sensitivity to lower energies. The lowest energy threshold is obtained using monoscopic analysis of data taken with CT5, providing access to gamma-ray ene
The addition of a 28 m Cherenkov telescope (CT5) to the H.E.S.S. array extended the experiments sensitivity to lower energies, providing new opportunities for studies of Active Galactic Nuclei (AGNs) with soft intrinsic spectra and at high redshifts.
Very high energy (VHE, $E>$100 GeV) $gamma$-ray flaring activity of the high-frequency peaked BL Lac object pg has been detected by the hess telescopes. The flux of the source increased by a factor of 3 during the nights of 2012 April 26 and 27 with
Axion-like particles are hypothetical new light (sub-eV) bosons predicted in some extensions of the Standard Model of particle physics. In astrophysical environments comprising high-energy gamma rays and turbulent magnetic fields, the existence of ax
Studying the temporal variability of BL Lac objects at the highest energies provides unique insights into the extreme physical processes occurring in relativistic jets and in the vicinity of super-massive black holes. To this end, the long-term varia