Kavli Affiliate: Dheeraj Pasham
| Summary:
We study AT2024kmq and AT2024lhc, two tidal disruption events (TDEs) with blue featureless spectra associated with high-mass black holes ($M_rm BHsim 10^8,M_odot$). Both events show optical precursors consistent with shock dissipation from stream self-intersection. Their X-ray emission is luminous ($L_rm Xsim 10^44,rm erg,s^-1$), 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 $f_νpropto ν^1-Γ$). 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 $tlesssim 2$,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 ($1.5<Γlesssim 3$) from the outset. We interpret this result as evidence that the soft-to-hard state transition in TDEs occurs at the critical threshold of $dotM_rm acc sim 0.03 dot M_rm Edd$ (similar to X-ray binaries), using the fact that the transition timescale predicted by simple disk theory scales with black hole mass as $t_rm trpropto M_rm BH^-3/4$.
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