The Origin Of γ-ray Emission From Supernova Remnants

Gamma rays are always connected with relativistic particles and are the powerful probe for studying the acceleration of relativistic particles and related physical pro-cesses. In the last decade, thanks to the development of the space-based and ground-based gamma-ray telescopes, more and more gamma-ray-bright supernova remnants (SNRs) were found in the GeV-TeV band. Making research on gamma-ray emission from SNRs one of frontiers in modern astrophysics, these observational results help advance our understanding of many physical processed, e.g., the particle acceleration in SNR shock, the magnetic amplification, the feedback of high energy particles on the SNR evolution, the century puzzle of cosmic rays (CRs)’origin, etc. The char-acteristics of gamma-ray emission from SNRs play an essential role in probing these issues, we therefore need to answer the fundamental question:How are gamma rays produced in SNRs? Both electrons via inverse Compton and bremsstrahlung process (leptonic origin) and protons though inelastic proton-proton collision (hadronic origin) can generate gamma-ray emission in SNRs, but it is not easy to distinguish them. In this context, focusing on individual SNRs, we study the origin of gamma-ray emission.In chapter 1, we briefly review the background of the production and measurement mechanism of gamma-rays, the particle acceleration, and the recent observational and theoretical progress in study of gamma-ray-bright SNRs.In chapter 2, focusing on Tycho’s SNR and with multiwavelength observational evidences combined, we suggest that the gamma-ray emission arise from the interac-tion between the shock and molecular clouds and present a strong argument for the en-ergy budget in the accelerated protons and their hadronic gamma-ray emission. Based on the theoretical model of particle acceleration and the comparison with other young historical SNRs, we present a hadronic explanation with a modest energy conversion efficiency of the order of 1%. With such an efficiency, we obtain a normal electron-to-proton ratio of the order of 102 and an average ambient density (4-12 cm-3) which is at least one order of magnitude higher than that derived from optical and X-ray obser-vations. This high gas density is consistent with the multi-band evidence of the pres-ence of a dense medium to the north-east of the Tycho SNR. Moreover, the SNR-cloud interaction at the northeastern boundary has been newly confirmed by our latest CO observations. The SNR-cloud association, in combination with the H labsorption data, helps to constrain the so-far controversial distance to Tycho and leads to an estimate of 2.5 kpc.In chapter 3, focusing SNR RX J1713.7-3946, we study the role of the diffu-sive protons in gamma-ray emission. We first present a simplified algorithm for Li & Chen’s (2010,2012) "accumulative diffusion" model for escaping protons. Con-sidering that the SNR shock propagates in a clumpy molecular cavity, We explore the role played by the diffusive relativistic protons in the gamma-ray emission and build up a two-zone model. We fit the broad-band data and apply Markov chain Monte Carlo method to constrain the model parameters. We find that the GeV emission are mainly from the outer emission zone where gamma-rays are produced in the interaction be-tween the cavity wall and energetic protons escaping from SNR shock, and the TeV emission mainly come from the inner emission region where gamma-rays are gener-ated via inverse Compton or the inelastic collision between the shocked clumps and the trapped protons. The two-zone model can also explain the TeV gamma-ray radial brightness profile that significantly stretches beyond the nonthermal X-ray emitting re-gion.In chapter 4, we apply the diffusive proton model to SNR G349.7+0.2 and nat-urally explain the GeV-TeV data with a broken power-law form, with the correction factor of slow diffusion of order χ～0.01-0.1. In this model, we consider two kinds of injection process:at any time, the protons escaping from the expanding shock surface have a power-law form or a (5-function form. Comparing the fitted results constrained via MCMC method with the CO observations, we find that whatever the injection pro-cess we adopt, the protons should have a slow diffusion around this SNR comparing with the mean diffusion coefficient in Galaxy disk. The correction factor is in the order of χ～0.01 in the power-law injection and χ～0.1 in the δ-function injection.In chapter 5, we focus on the TeV-bright shell-type SNRs including another two young SNRs and adopt a simple one-zone emission model to fit the radio, X-ray, and gamma-ray spectra. Based on the fitted results, we find that the electron spectrum hardens gradually with the expansion of the shocks. The evolution of the other model parameters appear to experience two distinct phases:in the early phase up to about 1000 yrs, the mean magnetic field decrease with time and the electron spectrum has a sharp high-energy cutoff; in the phase beyond 1000 yrs, the mean magnetic field does not change significantly and the high-energy cutoff of electron distribution broadens dra-matically. These results are consistent with the general paradigm of shock evolution in SNRs. However the hardening of the electron distribution with time and the mag-netic field evolution challenges the classical linear diffusive shock particle acceleration model.In chapter 6, we present the theoretical calculation for the other gamma-ray-bright SNRs observed by Fermi, including HESS J1731-347, SN 1006, RCW 103, Kes27 and Kes41, and rule out the possible gamma-ray emitting mechanism according to the model fitting. In those cases, we can see that the environment play an important role in clarification of the origin of the gamma-ray from SNRs.Last, we give a summary of our works and discuss the hot issues in the field of gamma-ray emission from SNRs, and also point out some works with further explo-ration in this field.