The Spectral Index Distribution of EGRET Blazars: Prospects forGLAST

The intrinsic distribution of spectral indices in GeV energies of gamma-ray-loud blazars is a critical input in determining the spectral shape of the unresolved blazar contribution to the diffuse extragalactic gamma-ray background, as well as an important test of blazar emission theories. We present a maximum likelihood method of determining the intrinsic spectral index distribution (ISID) of a population of γ-ray emitters that accounts for error in the measurements of individual spectral indices, and we apply it to EGRET blazars. We find that the most likely Gaussian ISID for EGRET blazars has a mean of 2.27 and a standard deviation of 0.20. In addition, we find some indication that FSRQs and BL Lac objects may have different ISIDs (with BL Lac objects being harder). We also test for spectral index hardening associated with blazar variability, for which we find no evidence. Finally, we produce simulated GLAST spectral index data sets and perform the same analyses. With improved statistics due to the much larger number of resolvable blazars, GLAST data will help us determine the ISIDs with much improved accuracy. Should any difference exist between the ISIDs of BL Lac objects and FSRQs or between the ISIDs of blazars in the quiescent and flaring states, GLAST data will be adequate to separate these ISIDs at a significance better than 3 σ.