Revisiting Analysis On Five Gamma-ray Globular Clusters With Fermi-LAT Pass8 Data

Globular clusters are roughly spherical systems of old stars which are bound by strong gravity.They are one of a new kind gamma-ray sources discovered Fermi LAT.There are about 30 globular clusters have been detected in gamma-rays,and the origin of the gamma-rays is still an open question.In this thesis,by using the latest Pass8 data(P8R3)of Fermi LAT,we analyze the gamma-ray emission from five globular clusters,NGC 5904,NGC 6397,M 15,NGC 6139 and NGC 6218.The gamma-ray photons from the first three objects have been detected,and the last two are potential gamma-ray sources.We use the v11r5p3 software package of Fermi Science Tools to analyze 10-year observation data(from August 4,2008 to October 31,2018),a new Galactic diffusion model and an extragalactic isotropic diffusion model are used in our analysis.The main results are as follows:(1)The confidence of the five sources is mostly improved.For the identified gamma-ray globular clusters,the confidence of NGC 5904 increased from 6.4?(TS=41)to 6.7?(TS=47);The confidence of NGC 6397 increased significantly from 7?(TS=47)to 10.6?(TS=113).The confidence of M 15 6.65?(TS = 43)is similar to that of the previous 6.9?(TS=47).For the two potential gamma-ray sources,NGC 6139 and NGC 6218,we showed that they do exist gamma-ray emision.The confidence of NGC 6139 is 12.4 ?(TS = 141)and NGC 6218 is 6.9 ?(TS=47).(2)The analysis of the light curve of these five sources show that they are steady gamma-ray sources and no time variation,and the energy spectrum can be fitted with a PowerLaw spectrum.The Flux of NGC 5904 is F??(3.83 ± 1.23)x 10-9 ph cm-2 s-1,and the index is ?= 2.29± 0.13;NGC 6397 are F? =(1.57 ± 0.78)x 10-8 ph cm-2 s-1 and ? = 2.97±0.37;M 15 are F? =(7.18±1.59)×10-9 ph cm-2 s-1 and ? = 2.64±0.10;NGC 6139 are F? =(2.76 ± 0.43)x 10-8 ph cm-2 s-1 and ? = 2.74 ± 0.10;NGC 6218 are F??(7.37 ± 2.75)× 10-8 ph cm-2 s-1 and ? = 2.49 ± 0.19?...