Search for Gamma-ray Line Features with Fermi-LAT Data and Development of DAMPE Gamma-ray Science Analysis Software

Many astrophysical and cosmological phenomena, such as the flat rotation curves of galaxies, the discrepancy between luminosity masses and kinematic masses of galaxy clusters, and the cosmic microwave background power spectrum, indicate the existence of a large amount of so-called dark matter (DM) in the Universe. It is known that none of the currently known standard model (SM) particles can be viable for the DM particle, thus the discovery of the DM may open a new window for our fundamental physics. However, though it is well established that the DM consists ∼26% of the total energy density of the current Universe, its nature is still far from clear since all the evidence or properties are inferred from gravitational effects. It is highly necessary to find non-gravitational evidences of the DM. One way to search for such non-gravitational evidences is the DM indirect detection, by which we search for the annihilation or decay products of DM, including photons, electrons and positrons, protons and antiprotons, and neutrinos and antineutrinos. Among all kinds of possible DM annihilation signals, monochromatic gamma-ray lines, generated by DM annihilating to double photons directly, play an important role in the DM indirect detections. Since no known astrophysical process could generate a line-like gamma-ray signal, it can be clearly discriminated from astrophysical backgrounds.

In the first chapter, we introduce some general background of the dark matter indirect detection.

In the second and third chapters, we introduce our works on searching for gamma-ray line emission originated from the dark matter annihilation in some nearby massive Galaxy Clusters and the Milky Way satellites, respectively. No compelling evidence for the line signal has been identified, and upper limits are set on the annihilation cross section of dark matter particles into mono-energetic gamma-rays. The only potential candidate is a ∼ 43 GeV line-like emission in the directions of 16 nearby massive Galaxy Clusters, and the possible dark matter interpretations of such a signal have been examined. In the fourth chapter, we have set the limits on the cross sections of dark matter annihilation to the gamma-ray lines with subhalo distributions in N-body simulations and Fermi-LAT (Large Area Telescope) data.

In the fifth chapter, we introduce some of our works in the continual signal search of gamma-ray DM, including the dependence of the Galactic GeV excess on the diffuse emission background, the tentative GeV emission in the direction of dwarf spherical galaxy Tuc III, and the possibilities of 3FGL J2212.5+0703 and 3FGL J1924.8-1034 being dark matter subhalos.

In these years I have also been concentrated on developing the gamma-ray science analysis software of DAMPE (Dark Matter Particle Explorer) satellite, the first space based high energy cosmic ray and γ-ray detector of China. In the sixth chapter, I introduce the mathematical foundation, the framework, and the current status of this software. Moreover, some preliminary results obtained in the DAMPE gamma-ray data analysis by this software are shown. The DAMPE results are well consistent with Fermi-LAT, indicating that not only our software is valid for science analysis but also all the aspects of DAMPE satellite are in good conditions. We suggest that our understanding of the dark matter can be greatly improved with the help of DAMPE, Fermi-LAT, HERD (High Energy cosmic Radiation Detection), GAMMA-400, and other high energy detectors.