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 ($M_{\rm BH}\sim 10^8\,M_\odot$). Both events show optical precursors consistent with shock dissipation from stream self-intersection. Their X-ray emission is luminous ($L_{\rm X}\sim 10^{44}\,{\rm erg\,s^{-1}}$), highly variable (with minimum observed variability timescales of 1.3\,hr and 4.8\,hr for factor of $\sim3$ flux changes), long-lasting ($>1\,\rm yr$), emerging no later than the optical peak, and well characterized by power-laws with $1.7<Γ<3$ (where $f_ν\propto ν^{1-Γ}$). The X-ray properties and radio non-detections support a compact corona ($\lesssim 10 r_{\rm g}$) 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 $M_{\rm BH}$ distributions in TDEs with different X-ray spectral evolution properties: low-mass black holes ($M_{\rm BH} \sim 10^6 M_\odot$) remain soft ($Γ>4$) 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 ($1.5<Γ\lesssim 3$) from the outset. We interpret this result as evidence that the soft-to-hard state transition in TDEs occurs at the critical threshold of $\dot{M}_{\rm acc} \sim 0.03 \dot M_{\rm Edd}$ (similar to X-ray binaries), using the fact that the transition timescale predicted by simple disk theory scales with black hole mass as $t_{\rm tr}\propto M_{\rm BH}^{-3/4}$.

Figures (17)

• Figure 1: UV, optical and X-ray evolution of AT2024lhc. UV and optical light curves are shown in the upper panel, with epochs of optical spectroscopy marked with letter "S", and the epoch of HST spectroscopy marked with a vertical dotted line. Solid points represent detections above 3$\sigma$ in the optical and above 2$\sigma$ in the UV; semitransparent downward triangles indicate 3$\sigma$ upper limits. The observed 0.3--10 keV X-ray light curves are shown in the middle panel, with epochs of radio continuum observations marked with letter "R". The evolution of the X-ray power-law photon index $\Gamma$ is shown in the lower panel.
• Figure 2: UV, optical and X-ray evolution of AT2024kmq. We only indicate epochs of optical spectroscopy and radio observations first presented in this work, see Ho2025 for earlier observations.
• Figure 3: Early-phase optical light curves of AT2024kmq and AT2024lhc in flux space, showing the precursor emission detected in both events. Solid and semitransparent points represent $>3\sigma$ detections and other observations, respectively. The dashed horizontal line at zero is the average value pre-transient.
• Figure 4: Optical spectra of AT2024kmq obtained after October 2024 (see Ho2025 for earlier epochs). Strong telluric features in the ALFOSC spectrum are masked. No prominent broad emission lines characteristic of TDEs are detected.
• Figure 5: Optical spectroscopic observations of AT2024lhc. All epochs remain spectroscopically featureless (see Figure \ref{['fig:24lhc_opt_spec_sub']}).