2011ApJ...735..108C

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 models. (1) The synchrotron peak luminosities (L_s) significantly correlate inversely with the synchrotron peak frequencies (ν_s), L_s∝ν^{–0.44±0.11}_s, which is consistent with the blazar sequence. In addition to the correlation, there are some blazars showing low ν_s and low L_s. To study the relation between these low-ν_s-low-L_s blazars and the blazar sequence, we present correlations of the parameter L_sν^1/4_s with the ratio of Compton to synchrotron peak frequencies (r_Cs≡ ν_C/ν_s) and with the ratio of Compton to synchrotron luminosities (CD ≡ L_C/L_s ). The results indicate that both correlations are significant with a Pearson's probability for a null correlation of p = 0.0218 and p = 0.0286, respectively. This does not support the idea that the low-ν_s-low-L_s blazars are sources with less beaming. Another possibility, as suggested by Ghisellini & Tavecchio, is that these blazars have relatively lower black hole masses. To test this, we collect the black hole masses of 30 blazars from archives and find that the black hole mass correlates with the parameter L_sν^0.44 _s(p = 0.0344). Therefore, the black hole masses of low-ν_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 ν_ s_ and low L_s. This supports the above result. (2) The ratio r_Cscorrelates with CD significantly (p = 0.00375). The external Compton model can naturally explain this correlation, while the synchrotron self-Compton model cannot. This agrees with the findings of many authors that the EC process dominates the gamma-ray emission of flat spectrum radio quasars.