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

TL;DR

This study investigates the origin of soft X-ray emission in the tidal disruption event AT2021ehb by performing timing analyses on seven XMM-Newton epochs. The authors detect a soft X-ray lag of about $\tau \approx 500\ \mathrm{s}$ on timescales of $\sim 10^{4}\ \mathrm{s}$, with high coherence up to $\sim 3\times10^{-4}$ Hz, consistent with disk–corona reverberation. Energy and covariance spectra further support a disk–corona interpretation and reveal a prominent soft excess linked to the reverberated emission. The results imply a compact corona and relativistic disk reflection in AT2021ehb and offer a plausible mechanism for the rapid post-peak X-ray decline via corona cooling driven by magnetic energy loss, highlighting the dynamic disk–corona geometry in TDEs.

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 about 10^4 s time-scales, and measure a soft lag of about 500 s. This behaviour 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 about 200 eV) is difficult to reconcile with thermal emission from a standard accretion disk around a about 10^7 Msun 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.

Figures (3)

• Figure 1: Panel a: X-ray luminosity of AT2021ehb in the 0.3–10 keV band. Panels b–e show the evolution of the spectral parameters, where $T_{\rm in}$ denotes the inner-disk temperature and $R_{\rm in}^{*}$ refers to the apparent inner-disk radius derived from the diskbb model, defined as the true inner-disk radius multiplied by $\sqrt{\cos i}$ (where $i$ is the disk inclination), while $\Gamma$ and $f_{\rm sc}$ represent the photon index of the comptonized power-law component and the fraction of seed photons scattered into it. In panels b–e, the red star denotes the mean values derived from the spectral fits to the seven XMM-Newton observations, while the blue points are taken from Yao2022.
• Figure 2: Top panel: Soft (0.3–0.7 keV) versus hard (0.9–10 keV) lags as a function of temporal frequency. By convention, negative lags correspond to the soft band lagging behind the hard band. Variability on time-scales of 10$^{4}$ s shows a soft lag of approximately 500 s. Bottom panel: Coherence as a function of frequency between the 0.3–0.7 and 0.9–10 keV bands. The blue-shaded region highlights the frequency range in which the soft band lags the hard band, during which the coherence remains close to 1. The blue-shaded frequency range is used to construct the energy–lag spectrum and the covariance spectrum. All statistical errors correspond to the 90% confidence level.
• Figure 3: Energy-dependent variability functions. The red points show the energy–lag spectrum, and the black points show the covariance spectrum. The grey points represent the time-averaged pn spectrum, while the best-fitting model with the diskbb component removed is shown as a gray curve. The orange-shaded region marks the energies at which the soft band lags the reference band, corresponding closely to the energy range where the diskbb component contributes significantly in the time-averaged spectrum. All statistical errors correspond to the 90% confidence level.