2025ApJ...994..166M

X-ray emission arising from active galactic nucleus (AGN) activity may potentially mimic the expected emission of tidal disruption events (TDEs). Ongoing and upcoming wide-field X-ray surveys will detect thousands of TDE-like sources, and classifying them securely as TDEs or AGNs is a challenging task. To this aim, we measure the average X-ray variability of AGNs and derive a threshold of maximum variation as a function of time separating the TDEs from AGN flares. For the comparison between TDE and AGN X-ray variability, we cross-match the publicly available XMM-Newton and Swift-XRT point-source catalogs with the Million Quasars Catalog and optically selected TDEs. Then we compute the X-ray structure function (SF) and maximum variability of the AGN and TDE samples. The X-ray SF of AGNs has a power-law index γ ∼ 0.11-0.14 when fitted with a simple power-law model. However, the SF of AGNs is best described by a broken power law or a power exponential model with a damping timescale τ = 950 ± 300 days. The maximum variability comparison between TDE and simulated AGN light curves indicates they have a similar order of variation on a timescale of less than 20 days. However, at a longer timescale of ∼20 days or more, the large-scale variations expected from power-law-like decay in TDEs become less frequent in AGNs. Furthermore, we compare the maximum variability of eROSITA TDE candidates with AGNs, finding that many of the eROSITA-DE TDE candidates are consistent with flares from AGNs and may not be TDEs.