Fainter harder brighter softer: a correlation between αox, X-ray spectral state, and Eddington ratio in tidal disruption events
T. Wevers
Abstract
Abstract 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 αox, L$_{2\ \rm keV}$, and fEdd,X) as a function of the Eddington ratio. We report strong positive correlations between αox and fEdd,bol; fEdd,X and fEdd,UV; and an anticorrelation for L$_{2\ \rm keV}$ and fEdd,UV. TDEs at high fEdd,bol have thermal dominated X-ray spectra and high (soft) αox, whereas those at low fEdd,bol show a significant power-law contribution and low (hard) αox. Similar to X-ray binaries and active galactic nuclei, the transition between X-ray spectral states occurs around fEdd,bol ≈ 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 fEdd,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.