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Similarity of Jet Radiation between Flat Spectrum Radio Quasars and GeV Narrow-Line Seyfert 1 Galaxies: A Universal $δ$-$L_{\rm c}$ Correlation

By modeling the broadband spectral energy distributions (SEDs) of a typical flat spectrum radio quasar (FSRQ, 3C 279) and two GeV narrow-line Seyfert 1 galaxies (NLS1s, PMN J0948+0022 and 1H 0323+342) in different flux stages with the one-zone leptonic models, we find a universal correlation between their Doppler factors ($δ$) and peak luminosities ($L_{\rm c}$) of external Compton scattering bumps. Compiling a combined sample of FSRQs and GeV NLS1s, it is found that both FSRQs and GeV NLS1s in different stages and in different sources well follow the same $δ$-$L_{\rm c}$ correlation. This indicates that the variations of observed luminosities may be essentially due to the Doppler boosting effect. And the universal $δ$-$L_{\rm c}$ relation between FSRQs and GeV NLS1s in different stages may be further evidence that the particle acceleration and radiation mechanisms for the two kinds of sources are similar. In addition, by replacing $L_{\rm c}$ with the observed luminosity in the Fermi/LAT band ($L_{\rm LAT}$), this correlation holds, and it may serve as an empirical indicator of $δ$. We estimate the $δ$ values with $L_{\rm LAT}$ for 484 FSRQs in the Fermi/LAT Catalog and they range from 3 to 41, with a median of 16, which are statistically consistent with the values derived by other methods.