Abstract The effect of the observed continuum emitted from a relativistic jet on the measurement of the full width at half maximum (FWHM) of an emission line is analyzed. If the jet contribution is not properly subtracted, the FWHM of the line could seem narrower than it should. The cases of an emission line detected in BL Lac objects and γ-ray Narrow-Line Seyfert 1 galaxies (γ-NLS1s) are addressed. It is shown that the smallness of the observed FWHM of the Lyα lines detected in three well-known BL Lacs is an effect due to the combined action of both the relativistic jet and a weak accretion disk. Once the Doppler boosting effect of the jet continuum is removed, the intrinsic FWHM values of the lines are found to be in the usual range. By contrast, the narrow permitted lines in γ-NLS1s are really narrow, since the disk and the lines are much more powerful. This also confirms that γ-NLS1 is really a new class of γ-ray emitting AGN, different from blazars and radio galaxies.
Keywords line: profiles — galaxies: jets — BL Lacertae Objects: general — galax- ies: Seyfert
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