The Study Of Supernova Remnants And Starburst Galaxy With Fermi-LAT Data

Since the discovery of cosmic rays(CRs)in 1912,the origin of CRs is still uncer-tain.It is widely accepted that the supernova remnants(SNRs)are the main accelerators of Galactic CRs.It is difficult for us to detect the CRs from these accelerators directly owing to the Galactic magnetic filed.However,relativistic CRs could produce gamma-rays through the interactions with background photons and ambient materials.With the development of ground-based and space-based gamma-ray telescopes,more and more SNRs and other objects are detected in the gamma-ray band,which provides a good opportunity for us to explore the origin of CRs and study the particle acceleration the-ory.Our works mainly focus on SNRs,starburst galaxies and TeV unidentified sources with Fermi-LAT observations to study the radiation mechanisms and the origin of their gamma-ray emission.HESS J1731-347 is a TeV shell-type SNR.We searched for the GeV emission of it using 9 yr of Fermi-LAT data.We found a slightly extended GeV source in the direction of HESS J1731-347.The gamma-ray characteristics of HESS J1731-347 are similar to other TeV shell-type SNRs,including RX J1713.7-3946,RX J0852-4622,RCW 86,and SN 1006.A pure leptonic model can account for the wide-band SED of HESS J1731-347.If the hadronic process is adopted to explain its gamma-ray emission,a very hard(?1.6)proton spectrum is required.In addition,the energy budget of CR protons may be a problem,given the potential low gas density environment implied by the lack of thermal X-ray emission.The Circinus galaxy is a nearby composite starburst/active galactic nucleus(AGN)system.In this work,we re-analyzed the GeV emission from Circinus with 10 yr of Fermi-LAT Pass 8 data.Our 0.1-500 GeV luminosity is several times lower than that reported in previous literature,which is roughly in compliance with the empirical rela-tion for star-forming and Local Group galaxies.And this result indicates that the GeV emission of Circinus may be reproduced by the interaction between cosmic rays and the interstellar medium.However,marginal evidence for variability of the gamma-ray emission is found in the timing analysis,which may indicate the activity of an AGN jet.More Fermi-LAT data and future observation of CTA are required to fully reveal the origin of its gamma-ray emission.HESS J1857+026 was discovered as a very-high-energy gamma-ray source without clear identified counterpart.The GeV emission from HESS J1857+026 region was also detected with Fermi-LAT data.We re-analyzed the region around HESS J1857+026 with 11 yr of Fermi-LAT observation data and discussed the nature of this source.We found that the GeV emission of it spatially coincides well with MAGIC J 1857.2+0263,which may be a PWN powered by PSR J1856+0245.In addition,there is an obvious spectral upturn in several GeV,indicating different origin of gamma-ray emission.If HESS J1857+026 is a PWN,it is nature to argue that the gamma-ray emission below several GeV is from the pulsar.However,the gamma-ray emission of HESS J1857+026 in higher and lower energy band could also origin from SNR and the interaction between the escaped CRs from SNR and molecular cloud,respectively.Both the leptonic and hadronic model can fit the multi-wavelength observation data.Future multi-wavelength observations,especially the radio and X-ray observations,are important to constrain the nature of HESS J1857+026.