Extragalactic Gamma‐Ray Background

The origin of the extragalactic gamma‐ray background (EGRB) is an important open issue in the gamma‐ray astronomy. There are many theories about the origin of EGRB: (1) some truly diffuse processes, such as dark matter (DM) annihilation or decay, which can produce gamma rays; (2) gamma rays produced by energetic particles accelerated through induced shock waves during structure formation of the universe; (3) a lot of unidentified sources, including normal galaxies, starbursts and active galactic nuclei (AGNs), contain a large number of energetic particles and can emit gamma rays. Among various extragalactic sources, blazars including flat spectral radio quasars (FSRQs) and BL Lac objects are one of the most possible sources for EGRB. As continuous accumulation of the data observed by the Fermi Gamma‐Ray Space Telescope, it is possible to directly construct gamma‐ray luminosity function (GLF) of the blazars involving evolution information. In this chapter, based on the largest clean sample of AGNs provided by Fermi Large Area Telescope (LAT), we mainly study blazar\u27s GLFs and their contribution to EGRB. In our study, we separately construct GLFs of FSRQs and BL Lacs and then estimate the contributions to EGRB, respectively. Further, we discuss the diffuse gamma ray from other astrophysical sources and the other possible origins of the EGRB

The estimate of emission region locations of {\it Fermi} FSRQs

We study the locations of emission regions through modelling the quasi-simultaneous multi-frequency spectral energy distributions of 21 {\it Fermi} flat spectrum radio quasars (FSRQs) in the frame of a multi-component one-zone leptonic model. In our calculations, we take the detailed broad line region (BLR) structure into account and discuss the effect of the uncertainty of the BLR structure on constraining the location of the emission regions for each FSQR, meanwhile we also include both the internal and external absorptions. Our results indicate that (1) the contribution of external Compton-BLR component is important to $\gamma$-ray emission, and the energy density of external target photon fields depends on the location of the emission region, which can be derived through reproducing the observed $\gamma$-ray emission, and (2) the emission regions of FSRQs with relative low accretion disk luminosity lie in the region of $7.9\times10^{16}$\ --\ $1.3\times10^{18}$\ cm (300 -- 4300 Schwarzschild radii) from central black hole, and for FSRQs with high accretion disk luminosity, the emission regions locate in the larger region of $2.6\times10^{17}$\ --\ $4.2\times10^{18}$\ cm (300 -- 5600 Schwarzschild radii).Comment: 10 pages, 10 figures, 1 table. Accepted for publication in PAS

Evolution of High-Energy Particle Distribution in Mature Shell-Type Supernova Remnants

Multi-wavelength observations of mature supernova remnants (SNRs), especially with recent advances in gamma-ray astronomy, make it possible to constrain energy distribution of energetic particles within these remnants. In consideration of the SNR origin of Galactic cosmic rays and physics related to particle acceleration and radiative processes, we use a simple one-zone model to fit the nonthermal emission spectra of three shell-type SNRs located within 2 degrees on the sky: RX J1713.7-3946, CTB 37B, and CTB 37A. Although radio images of these three sources all show a shell (or half-shell) structure, their radio, X-ray, and gamma-ray spectra are quite different, offering an ideal case to explore evolution of energetic particle distribution in SNRs. Our spectral fitting shows that 1) the particle distribution becomes harder with aging of these SNRs, implying a continuous acceleration process, and the particle distributions of CTB 37A and CTB 37B in the GeV range are harder than the hardest distribution that can be produced at a shock via the linear diffusive shock particle acceleration process, so spatial transport may play a role; 2) the energy loss timescale of electrons at the high-energy cutoff due to synchrotron radiation appears to be always a bit (within a factor of a few) shorter than the age of the corresponding remnant, which also requires continuous particle acceleration; 3) double power-law distributions are needed to fit the spectra of CTB 37B and CTB 37A, which may be attributed to shock interaction with molecular clouds.Comment: Accepted for publication in The Astrophysical Journal, 11 pages, 3 figures, 1 tabl