AT2024lhc and AT2024kmq in the landscape of featureless tidal disruption events

Abstract

We study AT2024kmq and AT2024lhc, two tidal disruption events (TDEs) with blue featureless spectra associated with high-mass black holes ($M_{\mathrm{BH}}\sim 10^8{M}_{\odot }$). Both events show optical precursors consistent with shock dissipation from stream self-intersection. Their X-ray emission is luminous ($L_{\mathrm{X}}\sim 10^{44}\mathrm{erg}{\mathrm{s}}^{-1}$), highly variable (with minimum observed variability timescales of 1.3\,hr and 4.8\,hr for factor of $\sim 3$ flux changes), long-lasting ($>1\mathrm{yr}$), emerging no later than the optical peak, and well characterized by power-laws with $1.7<\hat{I}“<3$ (where $f_{\hat{I}}½\propto \hat{I}{½}^{1-\hat{I}“}$). The X-ray properties and radio non-detections support a compact corona ($\lesssim 10r_{\mathrm{g}}$) 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 $M_{\mathrm{BH}}$ distributions in TDEs with different X-ray spectral evolution properties: low-mass black holes ($M_{\mathrm{BH}}\sim 10^6{M}_{\odot }$) remain soft ($\hat{I}“>4$) within $t\lesssim 2$\,yr, intermediate masses ($\sim 10^7{M}_{\odot }$) transition from soft to hard at $\sim$1 yr, while high masses ($\sim 10^8{M}_{\odot }$) are hard ($1.5<\hat{I}“\lesssim 3$) from the outset. We interpret this result as evidence that the soft-to-hard state transition in TDEs occurs at the critical threshold of ${\dot{M}}_{\mathrm{acc}}\sim 0.03{\dot{M}}_{\mathrm{Edd}}$ (similar to X-ray binaries), using the fact that the transition timescale predicted by simple disk theory scales with black hole mass as $t_{\mathrm{tr}}\propto M_{\mathrm{BH}}^{-3/4}$.

24 pages, 16 figures, submitted to MNRAS