Study Of The Dark Matter Particles And The Active Galactic Nucleus Radiation Physics With Gamma-Ray Observations

In recent years,great progress has been made in gamma-ray observations from both space-based and ground-based telescopes.These telescopes have accumulated abundant observational data,allowing us to explore the underlying physics of the observed radiations.Astroparticle researches of the dark matter indirect detection and the high-energy cosmic rays depend on gamma-ray observations.In this thesis,we review the frontiers of these researches and describe in detail our work on searching for dark matter particles and studying the radiation physics of active galactic nuclei（AGN）.Beyond the standard model of particle physics,many dark matter particle candidates have been proposed.Among them,WIMPs（Weakly Interact Massive Particles）are the most popular candidate with masses around the Ge V to Te V range.They can produce gamma-ray photons via annihilation or decay.If WIMPs annihilate or decay directly into photons,monochromatic gamma-ray spectral lines can be produced.Such lines are smoking-gun evidence for the existence of WIMPs if detected.Using 13 years of Fermi-LAT and 3 years of DAMPE gamma-ray observations,we search for the spectral lines expected from the galactic dark matter halo.Four regions of interest（ROIs）are adopted centering at the Sgr A*,with the extension of 16,40,86,and 150 degrees respectively.We employ the sliding energy window method to perform the search and the Fermi-LAT and DAMPE data are fitted jointly in each window.We do not find any line structures thus the upper limit of the velocity-averaged cross section??_γγand the lower limit of the decay timescale_γνcan be derived,which are improved compared to previous limits.Considering that WIMPs are denser near the Sgr A*with a spike density profile,we also constrain??_γγwith a 1-degree ROI from DAMPE data.In this case,the constraint depends on the spike parameters and the branching factor of annihilation.UCMHs（Ultracompact Mini Halo）are a type of dark matter sub-halo initiated by the density fluctuation in the early universe.UCMHs today are very compact predicted by the radial infall theory.Thus,WIMPs annihilation in it would be very intense,producing detectable standard model particles.Studies indicate that the 1.4 Te V sharp e⁺e^-excess observed by DAMPE may originate from such a UMCH near Earth.Since the e⁺e^-production in WIMPs annihilation is accompanied by gamma-ray photons,we perform a systematic search for this UCMH gamma-ray candidate using the Fermi-LAT observations.As result,we end up finding 14 of the candidates which are consistent with the UCMH density profile for explaining the 1.4 Te V excess.In addition,we study the abundance of UCMHs in our universe with the latest Fermi-LAT EGB（Extragalactic Gamma-ray Background）observations.Constraint of the abundance is derived which is 1 or 2 orders of magnitude stronger than previous results.Axion-like particles（ALPs）are another type of well-motivated dark matter candidate.Their propagation in the universe can be affected by the external magnetic field and they may convert to gamma-ray photons.In the Ge V energy band,ALP masses around the ne V range can be probed with gamma-ray observations of astrophysical objects.Using the latest Fermi-LAT observations of the radio galaxy NGC 1275 and BL Lac object PKS 2155-304,we search for ALP effects that ICMF（Intracluster Magnetic Field）and GMF（Galactic Magnetic Field）are considered and the method of quantifying the spectrum irregularity is employed.Our results suggest that no ALP effects statistically exist in the observations.Thus,we can exclude ALP parameters at a 95%confidence level through simulations.On the TeV or higher energy band,the astrophysical origin of radiations is generally related to processes of particle acceleration and cooling under extreme physical conditions.The origin of high-energy cosmic rays is related closely to these processes and the AGN jets provide ideal targets for studying them.Based on the Te V excess observed in the intrinsic spectral of some BL Lac objects,we explore their origin with a single-zone leptohadronic model that includes proton-photon interactions and the pair cascade.This model effectively explains the observed Te V excesses,as well as the broadband energy distributions.We summarize that the next-generation gamma-ray telescopes like the LHAASO and CTA will provide further insights for studies of dark matter particles and radiation physics as well as the origin of cosmic rays.