Line and Continuum Variability in Active Galaxies

TL;DR

This study compares optical spectroscopic and photometric data of 18 active galactic nuclei over several years, revealing variability patterns in emission lines and continuum linked to black hole mass and narrow line region size.

Contribution

It provides new insights into the variability mechanisms of AGN emission lines and continuum, emphasizing the role of black hole mass and narrow line region size.

Findings

Variations in forbidden oxygen and hydrogen lines observed.

Continuum and line variability linked to black hole mass.

Narrow line region size estimated to be at least 10 light years.

Abstract

We compared optical spectroscopic and photometric data for 18 AGN galaxies over 2 to 3 epochs, with time intervals of typically 5 to 10 years. We used the Multi-Object Double Spectrograph (MODS) at the Large Binocular Telescope (LBT) and compared the spectra to data taken from the SDSS database and the literature. We find variations in the forbidden oxygen lines as well as in the hydrogen recombination lines of these sources. For 4 of the sources we find that, within the calibration uncertainties, the variations in continuum and line spectra of the sources are very small. We argue that it is mainly the difference in black hole mass between the samples that is responsible for the different degree of continuum variability. In addition we find that for an otherwise constant accretion rate the total line variability (dominated by the narrow line contributions) reverberates the continuum variability with a dependency $\Delta L_{line} \propto (\Delta L_{cont.})^{\frac{3}{2}}$. Since this dependency is prominently expressed in the narrow line emission it implies that the luminosity dominating part of the narrow line region must be very compact with a size of the order of at least 10 light years. A comparison to literature data shows that these findings describe the variability characteristics of a total of 61 broad and narrow line sources.