Abstract

X-ray variability is a well-established characteristic of active galactic nuclei (AGNs) known to correlate inversely with both the supermassive black hole mass (M-BH) and luminosity, although the degree of each remains a topic of debate. The potential of X-ray variability as a proxy for M-BH or for intrinsic L-X has led to proposals to use AGNs as standard candles to test cosmological models. However, the large intrinsic dispersion in these relations has limited their practical applications. In this work, we investigate the dependence of X-ray variability on physical properties of AGNs using a sample of 134 Seyfert 1 galaxies from the BAT AGN Spectroscopic Survey (BASS), which is the largest sample to date (it is more than three times larger than samples used in previous studies). Contrary to earlier findings, we observed that X-ray variability correlates with luminosity just as strongly as with M-BH. Furthermore, we still find no evidence of the expected anti-correlation between variability and the Eddington ratio, even when using refined bolometric luminosities from Spectral Energy Distribution (SED) fitting to compute the Eddington ratio. From a cosmological perspective, the increased sample size reduces the scatter in the log L-log sigma(2)(NXS) relation to similar to 0.63 dex - a significant improvement over previous results, but still too large to serve as competitive standard candles, when compared to Supernovae Ia (uncertainties on distances of similar to 5-10%) or the L-X - L-UV relation in quasars (uncertainties of 10-12%). We tested including the width of broad emission lines as additional parameters, but we found that this does not significantly lower the observed dispersion, contrary to previous studies on smaller samples. Finally, we discuss how future X-ray missions such as AXIS and NewAthena will improve this scenario by enabling precise variability measurements for thousands of AGNs up to redshift z similar to 3.