BASS-XL: X-ray variability properties of unobscured Active Galactic Nuclei

Abstract

We investigate the X-ray variability properties of Seyfert1 Galaxies belonging to the BAT AGN Spectroscopic Survey (BASS). The sample includes 151 unobscured (N$_{\rm H}<10^{22}$ cm$^{-2}$) AGNs observed with XMM-Newton for a total exposure time of ~27 Ms, representing the deepest variability study done so far with high signal-to-noise XMM-Newton observations, almost doubling the number of observations analysed in previous works. We constrain the relation between the normalised excess variance and the 2-10 keV AGN luminosities, black hole masses and Eddington ratios. We find a highly significant correlation between $\sigma^{2}_{NXS}$ and $M_{\rm BH}$, with a scatter of ~0.85 dex. For sources with high $L_{2-10}$ this correlation has a lower normalization, confirming that more luminous (higher mass) AGNs show less variability. We explored the $\sigma^{2}_{NXS}$ vs $M_{\rm BH}$ relation for the sub-sample of sources with $M_{\rm BH}$ estimated via the "reverberation mapping" technique, finding a tighter anti-correlation, with a scatter of ~ 0.65 dex. We examine how the $\sigma^{2}_{NXS}$ changes with energy by studying the relation between the variability in the hard (3-10 keV) and the soft (0.2-1 keV)/medium (1-3 keV) energy bands, finding that the spectral components dominating the hard energy band are more variable than the spectral components dominating in softer energy bands, on timescales shorter than 10 ks.