Flux-depending X-ray spectrum index of blazars

Based on a sample of 69 blazars, relationships between flux density and spectral index, and distributions of redshift and spectral index have been investigated for highly frequency peaked BL Lac (HBLs), lowly frequency peaked BL Lac (LBLs) and flat spectrum radio quasars (FSRQs) respectively. Our result shows that (1) an anti-correlation between flux density and spectral index is found for HBLs, there is no clear relation for LBLs or FSRQs, which suggest that the X-ray emission in HBLs are from synchrotron process while that for LBLs and FSRQs from synchrotron self-Compton process, (2) the X-ray spectral index has a consequence for the three subclasses with HBLs showing the softest X-ray spectrum and FSRQs the hardest X-ray spectrum and with LBLs the middle of HBLs and FSRQs.