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By modeling the broadband spectral energy distributions (SEDs) of a typicalflat spectrum radio quasar (FSRQ, 3C 279) and two GeV narrow-line Seyfert 1galaxies (NLS1s, PMN J0948+0022 and 1H 0323+342) in different flux stages withthe one-zone leptonic models, we find a universal correlation between theirDoppler factors ($\delta$) and peak luminosities ($L_{\rm c}$) of externalCompton 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 differentsources well follow the same $\delta$-$L_{\rm c}$ correlation. This indicatesthat the variations of observed luminosities may be essentially due to theDoppler boosting effect. And the universal $\delta$-$L_{\rm c}$ relationbetween FSRQs and GeV NLS1s in different stages may be further evidence thatthe particle acceleration and radiation mechanisms for the two kinds of sourcesare similar. In addition, by replacing $L_{\rm c}$ with the observed luminosityin the Fermi/LAT band ($L_{\rm LAT}$), this correlation holds, and it may serveas an empirical indicator of $\delta$. We estimate the $\delta$ values with$L_{\rm LAT}$ for 484 FSRQs in the Fermi/LAT Catalog and they range from 3 to41, with a median of 16, which are statistically consistent with the valuesderived by other methods.

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