Fast X-ray Variability from the Coronae of Supermassive Black Holes

TL;DR

This study provides the first systematic probe of short-timescale ($t_{\min}$ on the order of a few to tens of ks) X-ray variability in radio-quiet AGN by reanalyzing archival Chandra data for roughly $\approx$ $\!\!\!$ $3{,}000$ broad-line AGN from SDSS and DESI. Using a consistent light-curve extraction with multi-timescale binning and a $4\sigma$ significance criterion, the authors identify $14$ AGN with significant variability across $16$ observations and translate $t_{\min}$ into coronal-variable-region sizes $R_{\rm CVR}$, finding typical upper limits $\lesssim 10^{-4}$ pc and often comparable to or smaller than microlensing-inferred coronal extents. The results favor a highly inhomogeneous, magnetically dominated corona with localized, transient structures undergoing rapid energy dissipation, consistent with GRMHD expectations and magnetic reconnection scenarios. Gravitational and relativistic effects near the SMBH are discussed as modifiers of observed timescales and amplitudes, including potential Doppler boosting of compact flares. Looking ahead, future high-throughput X-ray missions such as AXIS, eXTP, and NewAthena will enable detections of faster and lower-amplitude variability, providing tighter constraints on coronal geometry and energization mechanisms around supermassive black holes.

Abstract

We present the first systematic study of short-timescale X-ray variability in radio-quiet active galactic nuclei (AGN), utilizing archival Chandra observations of approximately 3,000 broad-line AGN selected from the SDSS and DESI spectroscopic surveys. We identify 14 AGN exhibiting rapid (on timescales of tens of kiloseconds) X-ray flux variations by factors of two or more that are statistically significant ($p\le6\times10^{-4}$), indicative of fast coronal variability. By converting minimum variability timescales to light-crossing times, we place upper limits on the sizes of the variable coronal regions, finding typical scales of $\lesssim10^{-4}$~pc. The coronal variable region size upper limits of an AGN in our sample are found to be much smaller than the typical coronal sizes inferred from microlensing, suggesting that its corona is composed of localized, transient structures rather than smooth, homogeneous plasmas. Such efficient magnetic energy dissipation in compact volumes is consistent with expectations for magnetically dominated coronae and is supported by recent general relativistic magnetohydrodynamic simulations. Future high-throughput X-ray observatories will enable the detection of even faster coronal variability, providing direct constraints on the physical mechanisms driving plasma energization and flux fluctuation near supermassive black holes. Our results suggest that luminous AGN hosting massive black holes are prime targets for probing the small-scale structure and dynamics of AGN coronae.

Figures (5)

• Figure 1: 3 $\sigma$ upper limits of coronal variable region radius ($R_{\rm CVR}$) in units of r$_{g}$ (upper) and pc (lower) measured in this work (red) and coronal size measured by microlensing method Chartas2016 as a function of the source black hole mass.
• Figure 2: The luminosity variability and variability timescale of X-ray transients and variables, adopted from Polzin2023. We adopted rest frame time above half-maximum flux reported in Polzin2023 as the variability timescale. While sharing similar luminosity variability with TDEs and long-timescale AGN activity, the short-timescale variabilities of AGN coronae (blue hatched region) unfold on significantly shorter timescales, akin to those of some long GRBs.
• Figure 3: Light curves in 0.5--7 keV of the 16 observations of the 14 sources studied in this work. The background count rate has been normalized to the source region size. The two bins used to compute t$_{\rm min}$ are plotted in red points.
• Figure 4: Figure \ref{['fig:LC1']} (continued). J1238+6213 presents significant variability in three separate observations.
• Figure 5: Figure \ref{['fig:LC1']} (continued).