Magnetic Reconnection as a Potential Driver of X-Ray Variability in Active Galactic Nuclei

Abstract We present a systematic analysis on the X-ray variability in 13 bright quasars at z > 4.5, combining recent Swift observations from 2021 to 2023 and archival multiepoch observations. Upper limits of the luminosity measurements were included in the analysis by using the Kaplan–Meier estimator method. It is found that the high-redshift quasars exhibit X-ray variability on both short-term (hours to days) and intermediate-term (weeks to months) timescales, with short-term variability dominating the overall variation. A linear correlation exists between the global mean ( μ L 2 − 10keV ) and standard deviation ( σ L 2 − 10keV ) of X-ray luminosities, which is independent of the X-ray photon index and optical-to-X-ray spectral slope. The localized stochastic magnetic reconnection mechanism is strongly favored, which can naturally lead to a scale-invariant power-law energy distribution and satisfactorily explain the correlation. The σ – μ correlation parallels the well-documented rms–flux relation of low-redshift active galactic nuclei (AGNs), implying that the magnetic reconnection mechanism could drive short-timescale X-ray variability in both high- and low-redshift AGNs. The highest-redshift quasar in our sample, J142952+544717 ( z = 6.18), shows a luminosity distribution extending to 10 47 erg s −1 with a not conspicuous median luminosity. On the other hand, J143023+420436 ( z = 4.7), which hosts the most relativistic jet among known high-redshift blazars, is dominated in the high-luminosity regime (10 47 erg s −1 ), making it an ideal target for multiwavelength follow-up observations. J090630+693030 is found to have a rest-frame period of 182.46 days, and J143023+420436 has a period of 16.89 days; both could be explained by the global evolution of plasmoid chains, in which magnetic islands formed during reconnection may merge successively.