Fainter harder brighter softer: a correlation between $\alpha_{\rm ox}$ , X-ray spectral state and Eddington ratio in tidal disruption events

Abstract

We explore the accretion states of tidal disruption events (TDEs) using a sample of 7 X- ray bright sources. To this end, we estimate the relative contribution of the disk and corona to the observed X-ray emission through spectral modeling, 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 anti-correlation for L$_{2 keV}$ and f$_{Edd,UV}$. TDEs at high f$_{Edd,bol}$ have thermal dominated X-ray spectra and high (soft) ${\alpha}$_ox, 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$_{Edd,bol}$ ${\approx}$ 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. TDEs around the most massive supermassive black holes are observed in the hard state; this could indicate that TDE evolution is faster around more massive BHs.