The gamma-ray spectral index changes for blazars

Based upon Fermi 1FGL and EGRET 3EG samples,a sample including 79 blazars(53 FSRQs,26 BL Lacs) is presented.It is investigated that the correlations between the ratio of EGRET to Fermi blazars-ray flux densities and the spectral index differ for EGRET to Fermi blazars for three subclasses of high-frequency peaked BL Lacertae objects-HBL,low-frequency peaked BL Lacertae objects-LBL,and flat spectrum radio quasars-FSRQs.There is a consistent relationship between the ratio of the two γ-ray flux densities and the spectral index difference for the three subclasses.It suggests that the spectrum changed with the source brightness in the gamma-ray band.Both the spectral index difference and the correlation slopes follow a continuous sequence from FSRQs to LBLs to HBLs,which is consistent with the noted blazar sequence.