Line and continuum variability in active galaxies

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 ∆L line ∝ (∆L cont. ) 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.

A low-luminosity type-1 QSO sample

“…To estimate the reliability of the flux calibration, we calibrated the standard stars with each other. The resulting calibrations varied by a factor of 10-35%, most likely caused by seeing variations during the night (for a discussion of slit-loss corrections see Rashed et al 2015).…”

Experimental Indicators of Accretion Processes in Active Galactic Nuclei

“…In the following ∆ denotes differences between the two samples. Rashed et al (2015) could show that for an approximately constant accretion rate the total Narrow Line Region line variability reverberates the continuum variability. The entire data set points at a relation between the line (∆L line ) and continuum (∆L cont ) variability of ∆L line ∝(∆L cont ) 3/2 .…”

“…The spectra were compared with SDSS and literature data. Rashed et al (2015) report significant variations in the forbidden oxygen and the hydrogen recombination lines of several sources. The authors show that it is most likely the difference in black hole mass between the stronger (broad line spectrum) and less variable (narrow line spectrum) subsets of the sample that is responsible for the different strength in the continuum variability.…”

“…Using the MODS spectrograph at the Large Binocular Telescope (LBT), Rashed et al (2015) compared optical spectroscopic and photometric data for 18 AGN galaxies over 2 to 3 epochs, spanning over a time interval of typically 5 to 10 years. The spectra were compared with SDSS and literature data.…”

“…the brightness is centrally peaked. Here, dM dt are the accretion rates and ∆M is the median SMBH mass difference between dominantly narrow and broad line spectrum dominated sources of the two samples treated by Rashed et al (2015).…”