BASS

LV. Connecting X-ray variability with physical properties of active galactic nuclei and a new path to cosmological distances

¹ Dipartimento di Matematica e Fisica, Università degli Studi Roma Tre, via della Vasca Navale 84, I-00146 Roma, Italy
² INAF – Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, I-50125 Firenze, Italy
³ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monte Porzio Catone, Italy
⁴ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, I-40129 Bologna, Italy
⁵ Pontificia Universidad Católica de Chile, Instituto de Astrofísica, Casilla 306, Santiago 22, Chile
⁶ Instituto de Estudios Astrofísicos, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejército Libertador 441 Santiago, Chile
⁷ Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, Sendai 980-8578, Japan
⁸ Astronomical Institute, Tohoku University, Aramaki, Aoba-ku, Sendai, Miyagi 980-8578, Japan
⁹ Global Center for Science and Engineering, Faculty of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan
¹⁰ Cahill Center for Astronomy and Astrophysics, California Institute of Technology, Pasadena, CA 91125, USA
¹¹ Department of Physics, SRM University–AP, Amaravati 522240, India
¹² Eureka Scientific, 2452 Delmer Street, Suite 100 Oakland, CA 94602-3017, USA
¹³ Department of Physics and Astronomy, Georgia State University, 25 Park Place, Suite 605 Atlanta, GA 30303, USA
¹⁴ Korea Astronomy and Space Science Institute, 776 Daedeokdae-ro, Yuseong-gu, Daejeon 34055, Republic of Korea
¹⁵ European Space Agency (ESA), European Space Research and Technology Centre (ESTEC), Keplerlaan 1, 2201 AZ, Noordwijk, The Netherlands
¹⁶ Department of Physics, Yale University, P.O. Box 208120 New Haven, CT 06520, USA
¹⁷ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
¹⁸ Institute for Astronomy, University of Hawai’i, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
¹⁹ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, MS 169-224 Pasadena, CA 91109, USA
²⁰ School of Physics and Astronomy, Tel Aviv University, Tel Aviv, 69978, Israel
²¹ Instituto de Alta Investigación, Universidad de Tarapacá, Casilla 7D, Arica, Chile
²² Yale Center for Astronomy & Astrophysics and Department of Physics, Yale University, P.O. Box 208120 New Haven, CT 06520-8120, USA
²³ Department of Astronomy, University of Geneva, ch. d’Ecogia 16, 1290 Versoix, Switzerland
²⁴ Instituto de Estudios Astrofísicos, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejército Libertador 441 Santiago, Chile
²⁵ Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, PR China

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 6 November 2025
Accepted: 16 February 2026

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 σ_NXS² relation 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 ∼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 ∼ 3.