Long term Ultra-Violet Variability of Seyfert galaxies

TL;DR

This study investigates the long-term ultraviolet flux variability of fourteen Seyfert galaxies using IUE data from 1978 to 1995, revealing correlated flux variations across UV bands and links to luminosity and black hole mass.

Contribution

It provides the first comprehensive analysis of long-term UV variability in Seyfert galaxies, highlighting correlations with luminosity and black hole mass.

Findings

All sources showed UV flux variations with no wavelength dependence.

UV flux variations in NUV and FUV are correlated with no time lag.

Approximately 50% of sources exhibit a hardening spectral index when brightening.

Abstract

Flux variability is one of the defining characteristics of Seyfert galaxies, a class of active galactic nuclei (AGN). Though these variations are observed over a wide range of wavelengths, results on their flux variability characteristics in the ultra-violet (UV) band are very limited. We present here the long term UV flux variability characteristics of a sample of fourteen Seyfert galaxies using data from the International Ultraviolet Explorer acquired between 1978 and 1995. We found that all the sources showed flux variations with no statistically significant difference in the amplitude of UV flux variation between shorter and longer wavelengths. Also, the flux variations between different near-UV (NUV, 1850 - 3300 A) and far-UV (FUV, 1150 - 2000 A) passbands in the rest frames of the objects are correlated with no time lag. The data show indications of (i) a mild negative correlation of UV variability with bolometric luminosity and (ii) weak positive correlation between UV variability and black hole mass. At FUV, about 50% of the sources show a strong correlation between spectral indices and flux variations with a hardening when brightening behaviour, while for the remaining sources the correlation is moderate. In NUV, the sources do show a harder when brighter trend, however, the correlation is either weak or moderate.