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We present X-ray, UV, optical, and radio observations of the nearby($\approx78$ Mpc) tidal disruption event (TDE) AT2021ehb/ZTF21aanxhjv duringits first 430 days of evolution. AT2021ehb occurs in the nucleus of a galaxyhosting a $\approx 10^{7}\,M_\odot$ black hole ($M_{\rm BH}$ inferred from hostgalaxy scaling relations). High-cadence Swift and NICER monitoring reveals adelayed X-ray brightening. The spectrum first undergoes a gradual ${\rm soft}\rightarrow{\rm hard}$ transition and then suddenly turns soft again within 3days at $\delta t\approx 272$ days during which the X-ray flux drops by afactor of ten. In the joint NICER+NuSTAR observation ($\delta t =264$ days,harder state), we observe a prominent non-thermal component up to 30 keV and anextremely broad emission line in the iron K band. The bolometric luminosity ofAT2021ehb reaches a maximum of $6.0^{+10.4}_{-3.8}\% L_{\rm Edd}$ when theX-ray spectrum is the hardest. During the dramatic X-ray evolution, no radioemission is detected, the UV/optical luminosity stays relatively constant, andthe optical spectra are featureless. We propose the following interpretations:(i) the ${\rm soft }\rightarrow{\rm hard}$ transition may be caused by thegradual formation of a magnetically dominated corona; (ii) hard X-ray photonsescape from the system along solid angles with low scattering optical depth($\sim\,$a few) whereas the UV/optical emission is likely generated byreprocessing materials with much larger column density -- the system is highlyaspherical; (iii) the abrupt X-ray flux drop may be triggered by thethermal-viscous instability in the inner accretion flow leading to a muchthinner disk.

The Tidal Disruption Event AT2021ehb: Evidence of Relativistic Disk Reflection, and Rapid Evolution of the Disk-Corona System