2026ApJ...999..265B

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 2σ and systematically yield a lower BH mass (${\mathrm{log}}({M}_{{\rm{BH}}}/{M}_{\odot })=5.67\pm 0.09$) than inferred from host galaxy scaling ($\mathrm{log}({M}_{{\rm{BH}}}/{M}_{\odot })=6.14\pm 0.19$).