Letter to the Editor

Discovery of a soft X-ray lag in the tidal disruption event AT2021ehb

National Astronomical Observatories, Chinese Academy of Sciences Beijing 100101, China

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Received: 9 December 2025
Accepted: 14 January 2026

Abstract

In this Letter, we report the detection of soft X-ray time lags – i.e., variability in the softer photons lagging behind that in the harder photons – in seven XMM–Newton observations of the tidal disruption event (TDE) candidate AT2021ehb. We find correlated variability between the soft (0.3–0.7 keV) and hard (0.9–10 keV) bands on ∼10⁴ s timescales, and measure a soft lag of ∼500 s. This behavior is broadly consistent with the disk–corona reverberation scenario established in active galactic nuclei (AGNs). Together with the previously reported strong hard X-ray emission and broad Fe K line, our results suggest the presence of a compact corona and prominent relativistic disk reflection in AT2021ehb. The unusually high blackbody temperature (peaking at ∼200 eV) is difficult to reconcile with thermal emission from a standard accretion disk around a ∼10⁷ M_⊙ black hole, and may instead be analogous to the soft excess commonly observed in AGNs, whose physical origin remains debated. Finally, the measured lags offer a possible explanation for the rapid X-ray flux decline that occurred only three days after the peak, pointing to a scenario in which the corona cools following a sudden loss of the magnetic support required to sustain it.