Fainter harder brighter softer: a correlation between αox, X-ray spectral state, and Eddington ratio in tidal disruption events

We explore the accretion states of tidal disruption events (TDEs) using a sample of seven X-ray bright sources. To this end, we estimate the relative contribution of the disc and corona to the observed X-ray emission through spectral modelling, and assess the X-ray brightness (through alpha(ox), L-2 keV, and f(Edd,X)) as a function of the Eddington ratio. We report strong positive correlations between alpha(ox) and f(Edd,bol); f(Edd,X) and f(Edd,UV); and an anticorrelation for L-2 keV and f(Edd,UV). TDEs at high fEdd, bol have thermal dominated X-ray spectra and high (soft) aox, whereas those at low f(Edd,bol) show a significant power-law contribution and low (hard) alpha(ox). Similar to X-ray binaries and active galactic nuclei, the transition between X-ray spectral states occurs around f(Edd,bol) approximate to 0.03, although the uncertainty is large due to the small sample size. Our results suggest that X-ray surveys are more likely to discover TDEs at low f(Edd,bol), whereas optical surveys are more sensitive to TDEs at high Eddington ratios. The X-ray and optical selected TDEs have different UV and X-ray properties, which should be taken into account when deriving rates, luminosity, and black hole mass functions.