Spectral Properties of Fermi Blazars and their Unification Schemes

We use the distributions of spectral indices of a large homogenous sample of Fermi-detected blazars to re-investigate the relationship between flat spectrum radio quasars (FSRQs) and subclasses of BL Lac objects (BL Lacs). We compute the broadband synchrotron and Compton spectral indices from radio-to-X-ray and X-ray to gamma-ray bands, respectively. Analyses of our data show continuity in the distributions of the spectral indices from FSRQs to HSP through LSP and ISP subclasses of BL Lacs. We find from gamma-ray luminosity distribution that the jetted radio galaxies form the low-luminosity tail of the distribution, suggestive that the sequence can be extended to the young jetted galaxy populations. We observe a significant difference in the shape of Compton and synchrotron spectra: significant anti-correlation (r ~ -0.80) exists between the broadband Compton and synchrotron spectral indices. Furthermore, the broadband spectral indices vary significantly with redshift (z) at low redshift (z less than 0.3) and remain fairly constant at high (z greater than or egual to 0.3) redshift. The trend of the variations suggests a form of the evolutionary connection between subclasses of blazars. Thus, while the selection effect may be significant at low redshift, an evolutionary sequence can also be important. Our results are not only consistent with a unified scheme for blazars and their young jetted galaxy counterparts but also suggest that the broadband spectral sequence of blazars is not a secondary effect of redshift dependence.
Comment: 10 pages, 10 figures