Disk-to-Corona State Transition and Extreme X-ray Variability in the Tidal Disruption Event AT2019teq

TL;DR

AT2019teq captures a rare disk-to-corona transition in a tidal disruption event, displaying extreme X-ray variability and a long-lived corona that persists for at least $\gtrsim1100$ days. By combining spectral modeling with relativistic disk emission ($\text{kerrbb}$), coronal Comptonization ($\text{simpl}$), and timing analyses (excess variance and QPO search), the study derives black hole mass estimates clustered around $M_{\rm BH} \sim 5\times10^{5}\,M_{\odot}$, consistently lower than optical/UV-based masses. The tentative sub-milliHz QPO and the evolving coronal strength provide insight into accretion physics across mass scales, drawing intriguing parallels with X-ray binaries while highlighting potential differences in transition timescales for TDEs. The findings underscore the value of X-ray timing and spectroscopy in constraining black hole mass and accretion-state evolution in TDEs, and they motivate long-term, high-cadence monitoring with upcoming time-domain surveys.

Abstract

We present a five-year X-ray spectral and timing analysis of the optically selected Tidal Disruption Event (TDE) AT2019teq, which displays extreme variability, including order-of-magnitude changes in flux on minute-to-day timescales, and a rare late-time emergence of hard X-ray emission leading to the longest-lived corona in a known TDE. In one epoch, we detect sub-mHz quasi-periodic oscillations with significance tested via MCMC-based red-noise simulations (p $\leq 0.03$). AT2019teq exhibits a clear spectral evolution from a soft (blackbody-dominated) state to a hard (power-law-dominated) state, with a late-time radio brightening that may be associated with the state transition. We identify similarities between AT2019teq's evolution and X-ray binary soft-to-hard state transitions, albeit at higher luminosity and much faster timescales. We use the presence of both a disk-dominated and a corona-dominated state to apply multiple mass estimators from X-ray spectral and variability properties. These techniques are mutually consistent within $2σ$ and systematically yield a lower black hole mass ($\log(M_{BH}/M_{\odot}) = 5.67 \pm 0.09$) than inferred from host galaxy scaling ($\log(M_{BH}/M_{\odot})=6.14 \pm 0.19$).

Figures (9)

• Figure 1: Long term X-ray (upper) and UV/optical (lower) light curves of AT2019teq. X-ray flux is computed between $0.3$--$2$ keV for XMM-Newton (red), Swift XRT (blue), and NICER (beige). The ZTF $g$-band light curve is computed from difference photometry, converted to total flux using a reference magnitude and expressed at the pivot wavelength. The Swift UVOT uvw2 light curve is shown in purple.
• Figure 1: XMM-Newton background-subtracted science light curves, and 10--12 keV particle-background light curves. The dashed red lines mark the times beyond which the high-energy background remains above 10 ct s$^{-1}$; intervals beyond these times are discarded.
• Figure 2: XMM-Newton 60 s light curves for each observation. Light curves are extracted from 0.3--7.0 keV, except for the first two observations which are truncated at 2.0 and 4.0 keV, respectively, due to background domination at higher energies. The light curves show extreme variability, with order-of-magnitude changes in flux on timescales as short as 10 minutes.
• Figure 3: Upper panel:XMM-Newton EPIC-pn unfolded spectra and best-fit models for all observations. Color lightens with time. Lower panels: Normalized residuals $\Delta\chi =$(data - model)/error by observation. Spectra are binned for visual clarity.
• Figure 4: Time evolution of luminosity and coronal strength. Top to bottom: UVOT uvw2 luminosity (circles), X-ray luminosity (0.3--10 keV, stars) and scattering fraction (squares). Declining radio emission was detected around MJD 59900 after non-detections in early times, consistent with an outflow that peaked $\sim400$--$1000$ days post optical peak cendes22.