Study On High-energy Emission Of EHSP Blazars

Lepto-hadronic and Lepto-hadronic hybrid models have been successfully used to explain the very high energy gamma-ray emission of the most blazars.However,the recent discovery of a small number of extremely high-energy synchrotron peaked blazars(EHSP)poses a strong challenge to the standard lepton model.These special sources are ideal candidates which provide astrophysical sites for studying particle acceleration,relativistic particle cooling and indirect detection of extragalactic background light(EBL)and interstellar magnetic field(IGMF),and are important for the study of the possible origin of high-energy neutrinos.In this work,we have studied the energy spectrum and variability of EHSP,and established a model to try to explain the high-energy radiation of this type of sources.Firstly,we reduced the hard X-ray and GeV gamma-ray data of 4 EHSPs observed by NuSTAR and Fermi-LAT space satellites.We obtained their hard X-ray spectral indices,spectral indices and variability characteristics in the GeV energy region.We found that the GeV spectral indices of the four sources are consistent with the intrinsic spectral indices in the TeV energy band,but not with the spectral indices of the hard X-ray.The 4sources were not observed any GeV gamma-ray variability.Based on the results of spectrum and variability,two software ELMAG and CRPROPA 3 were applied to establish two models of cosmic ray propagation.In Model 1,the high energy gamma-ray was originated in the source region and propagating out to the observer.In model 2,the high energy gamma-ray comes from the electromagnetic cascade process of the high-energy protons generated from the source region.The results show that Model 2 can better fit the TeV spectrum,but cannot explain the GeV results,while the Model 1 can explain the GeV emission.Therefore,we conclude that the highenergy emission of EHSP may be dominated by two models.The gamma-ray produced from the source region contribute more to the GeV radiation of this kind of source,while the cascade process generated by protons dominates the TeV radiation.