The Study Of The Extragalactic Gamma-ray Background

The origin of the extragalactic gamma-ray background (EGB) is one of thefundamental unsolved problems in astrophysics. The EGB could originate fromeither truly difuse processes or from unresolved point sources. Truly difuseemission can arise from some processes such as the annihilation of dark matter,the emission of high energy particles accelerated by intergalactic shocks which areproduced during large scale structure formation, etc. Now, only a small fractionof the extragalactic gamma-ray emission is resolved into point sources.For a sample of possible γ-ray point sources that have not been detected bythe Fermi due to their faint fuxes or soft spectra, we can stack a large numberof them to improve the statistics. If their fuxes are not too faint, we can derivetheir mean fux and photon index by a maximum likelihood method.Applying the method to the Australia Telescope20GHz Survey sourcesundetected by the LAT, we found that these sources contribute about8.4%ofthe EGB. which are much smaller than the result (17%) of Ghirlanda et al.[51].But our method is more directly.Then, we use the method of image stacking to FIRST sources, and fnd thatthe FIRST sources undetected by the LAT can contribute about61.4%of theEGB. Considering the fux limit and incompleteness of the sample at the faintlimit, and the contribution of the fainter radio subsample to the EGB cannotbe ignored, we think that the point sources must contribute most of the EGB.The main contributor of the EGB maybe blazars, non-blazars AGNs, starburstgalaxies, and the sources cannot traced by the FIRST survey (including normalgalaxies and radio-quiet AGNs). Even though it is difcult to derive the fractionof each population contributing to the EGB using ours method alone, it is clearthat standard astrophysical scenarios can be invoked to explain the EGB.