Search For Gamma-ray Emission Of Globular Clusters With Fermi-LAT Data

Globular clusters (GCs) are the oldest spherical collection of stars (their typical ages are～1010 years) with low metallicity. One hundred and fifty-eight GCs were detected in the Milky Way Galaxy. Before 2008, only radio and/or optical data were available. In 2009 with the Fermi-LAT data, GC had been firstly detected in gamma-ray bands. Now GCs are known to be a significant kind of gamma-ray emitter. However, most GCs are at low Galactic latitudes and the contamination due to the gamma-ray emission from the Galactic plane is serious. It is thus challenging to detect them with Fermi-LAT. On the other hand, a larger sample is highly needed to better reveal the radi-ation mechanism of the GCs and to carry out reliable statistical studies of the gamma-ray emission properties. That is why in the past three years we have been concentrated on search for gamma-ray emission of GCs with the Fermi-LAT data.By the end of 2014, after analyzing the Pass 7Rep data of Fermi-LAT, we con-firmed the significant gamma-ray emission from 2MS-GC01 and IC 1257 and discov-ered gamma-ray emission from NGC 5904 and 6656 within their tidal radii. Also, strong evidence of significant gamma-ray emission was found from FSR 1735 (see the second chapter).In June 2015 the Fermi-LAT collaboration released their Pass 8 data. The remark-able improvements of Pass 8 data in comparison to Pass 7 include better energy mea-surements, improved angular resolution, and significantly increased effective area and so on. Such high quality data is highly valuable for GC search, with which we discov-ered significant gamma-ray emission from M 15 and NGC 6397, confirmed that NGC 5904 is a gamma-ray emitter and found evidence of gamma-ray emission from NGC 6218 and NGC 6139.The total numbers of millisecond pulsars in these gamma-ray GCs are estimated and we do not find evidence for gamma-ray pulsation or flux variability, implying that the detected gamma-ray emission were from a group of millisecond pulsars (rather than a single source), either due to the curvature radiation from magnetospheres or the inverse Compton scattering between the relativistic electrons/positrons and the background soft photons.In chapter 4, we present the work on high energy emission study of gamma-ray binary system PSR B1259-63/LS 2883 around its 2014 periastron. A GeV flare took place at 30 days after periastron and the gamma-ray flux during the flare increased by a factor of ten, with fast variability low to sub-daily timescale. X-ray results from Swift and XMM-Newotn showed the same profile as that during the 2010/2011 periastron.In chapter 5, we summarize our findings with some discussions.