Implications on the blazar sequence and inverse Compton models from Fermi bright blazars

Abstract

In this paper, we use the quasi-simultaneous spectra of Fermi bright blazars and Fermi detected narrow line Seyfert 1 (NLS1) to study the blazar sequence and inverse Compton (IC) models. I. The synchrotron peak luminosities (L_{s}) significantly inverse correlate with the synchrotron peak frequencies (\nu_{s}), L_{s}\propto\nu_{s}^{-0.44}, which is consistent with the blazar sequence. In addition to the correlation, there are some blazars showing low \nu_{s} and low L_{s}. To study the relation between these low \nu_{s} low L_{s} blazars and the blazar sequence, we present correlations of the parameter L_{s}\nu_{s}^{1/4} with the ratio of Compton to synchrotron peak frequencies (r_{Cs}\equiv\nu_{C}/\nu_{s}) and with the ratio of Compton to synchrotron luminosities (CD\equiv L_{C}/L_{s}). The results indicate that both correlations are significant with a Pearson's probability for null correlation of p=0.0218 and p=0.0286 respectively. This does not support the idea that the low \nu_{s} low L_{s} blazars are sources with less beaming. Another possibility, as suggested by Ghisellini & Tavecchio, is that these blazars have relative lower black hole masses. To test this, we collect the black hole masses of 30 blazars from archives, and find that the hole mass correlates with the parameter L_{s}\nu_{s}^{0.44} (p=0.0344). Therefore, the black hole masses of low \nu_{s} low L_{s} blazars are statistically small. The NLS1s are thought to have lower black hole masses. We find that the four NLS1s detected by Fermi have low \nu_{s} and low L_{s}. This supports the above result. II. The ratio r_{Cs} correlates with CD significantly (p=0.00375). The external Compton (EC) model can naturally explain this correlation, while synchrotron self Compton (SSC) model can not. This agrees with the findings of many authors that the EC process dominates the gamma-ray emission of Flat Spectrum Radio Quasars.