AT2024lhc and AT2024kmq in the landscape of featureless tidal disruption events

We study AT2024kmq and AT2024lhc, two tidal disruption events (TDEs) with blue featureless spectra associated with high-mass black holes (MBH ~ 108 M⊙). Both events show optical precursors consistent with shock dissipation from stream self-intersection. Their X-ray emission is luminous (LX ~ 1044 erg s−1), highly variable (with minimum observed variability timescales of 1.3 hr and 4.8 hr for factor of ~3 flux changes), long-lasting (>1\, yr), emerging no later than the optical peak, and well characterized by power-laws with 1.7 < Γ < 3 (where fν∝ν1 − Γ). The X-ray properties and radio non-detections support a compact corona (≲ 10rg) producing Comptonized X-ray emission. Using all published featureless TDEs, we find statistically significant bimodality in the distribution of their peak UV/optical blackbody luminosities and radii. We assemble a comparison TDE sample with early-time X-ray observations with eROSITA, in which we find different MBH distributions in TDEs with different X-ray spectral evolution properties: low-mass black holes (MBH ~ 106M⊙) remain soft (Γ > 4) within t ≲ 2 yr, intermediate masses (~107M⊙) transition from soft to hard at ~1 yr, while high masses (~108M⊙) are hard (1.5 < Γ ≲ 3) from the outset. We interpret this result as evidence that the soft-to-hard state transition in TDEs occurs at the critical threshold of Ṁacc ~ 0.03 ṀEdd (similar to X-ray binaries), using the fact that the transition timescale predicted by simple disk theory scales with black hole mass as tₜᵣ∝ MBH-3/4.