Disk-to-corona State Transition and Extreme X-Ray Variability in the Tidal Disruption Event AT2019teq

Abstract We present a 5 yr 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 submillihertz quasiperiodic oscillations with significance tested via Markov Chain Monte Carlo based red-noise simulations ( p ≤ 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 the disk-dominated and corona-dominated states to apply multiple mass estimators from X-ray spectral and variability properties. These techniques are mutually consistent within 2 σ and systematically yield a lower BH mass ( log ( M BH / M ⊙ ) = 5.67 ± 0.09 ) than inferred from host galaxy scaling ( log ( M BH / M ⊙ ) = 6.14 ± 0.19 ).