On the emission lines in active galactic nuclei with relativistic jets

TL;DR

This paper investigates how relativistic jets affect emission line measurements in active galactic nuclei, revealing that proper subtraction of jet emission is crucial for accurate line width estimation and confirming gamma-NLS1s as a distinct AGN class.

Contribution

It demonstrates the impact of jet continuum on emission line FWHM measurements and clarifies the intrinsic properties of lines in BL Lacs and gamma-NLS1s, establishing their differences.

Findings

Jet contribution can cause underestimation of FWHM if not properly subtracted.

Intrinsic FWHM of Ly alpha lines in BL Lacs are within normal ranges after correction.

Gamma-NLS1s have genuinely narrow lines, confirming their distinct nature.

Abstract

The effect of the observed continuum emitted from a relativistic jet on the measurement of the full width 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 what it should be. The cases of emission line detected in BL Lac objects and gamma-ray Narrow-Line Seyfert 1 galaxies (gamma-NLS1s) are addressed. It is shown that the smallness of the observed FWHM of the Ly alpha 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 disc. Once removed the Doppler boosting of the jet continuum, the intrinsic FWHM of the lines are found to be in the usual range. Instead, the narrow permitted lines in gamma-NLS1s are really narrow, since the disc and the lines are much more powerful. This also confirms that gamma-NLS1 is really a new class of gamma-ray emitting AGN, different from blazars and radio galaxies.