Utilizing Maximum Variability to Discern TDE Emission from AGN Flares

TL;DR

This study develops a variability-based method to distinguish TDEs from AGN flares in X-ray surveys by analyzing their maximum variability patterns over different timescales.

Contribution

It introduces a threshold of maximum X-ray variability as a function of time to differentiate TDEs from AGN activity, validated through cross-matched catalogs and variability analysis.

Findings

AGN X-ray variability follows a broken power-law with a damping timescale of ~950 days.

TDEs exhibit larger-scale variations over ~20 days, less common in AGNs.

Many eROSITA TDE candidates are consistent with AGN flares, not true TDEs.

Abstract

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 $\gamma\sim0.11-0.14$ 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 time scale $\tau=950\pm300$ days. The maximum variability comparison between TDE and simulated AGN light curves indicates they have a similar order of variation on a time scale of less than 20 days. However, at a longer time scale of $\sim20$ 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 AGN, finding that many of the eROSITA-DE TDE candidates are consistent with flares from AGNs, and may not be TDEs.