Comparison of the X-ray emission from Tidal Disruption Events with those of Active Galactic Nuclei

One of the main challenges of current tidal disruption events (TDEs) studies is that emission arising from AGN activity may potentially mimic the expected X-ray emission of TDEs. This makes it difficult to cleanly disentangle these two components without significant multiwavelength follow up. Here we compare the X-ray properties of TDEs with those of AGN to determine a set of characteristics which would allow us to discriminate between flares arising from these two objects. We find that at peak, TDEs are brighter than the population of AGN found at similar redshifts. However highly variable AGN can produce flare emission with a similar order of magnitude increase compared to preflare upperlimits as those seen arising from X-ray TDEs. Nevertheless, compared to AGN, TDEs decay significantly more monotonically, and their emission exhibits little to no variation in spectral hardness as a function of time. We also find that X-ray TDEs are less absorbed, and their emission is much softer than the emission detected from AGN found at similar cosmological distances. We derive the X-ray luminosity function (LF) for X-ray TDEs using the events classified as a X-ray or likely X-ray TDE by Auchettl et al. (2016). Interestingly, our X-ray LF matches closely the theoretically derived LF by Milosavljevi\'c et al. (2006) which assumes a higher TDE rate currently estimated from observations. Using our sample of X-ray TDEs and the results of Stone & Metzger (2016) we estimate a TDE rate of $(0.7-4.7)\times10^{-4}$ yr$^{-1}$ per galaxy, higher than current observational estimates. We find that TDEs can contribute significantly to the LF of AGN for $z\lesssim0.4$, while we find no evidence that TDEs influence the growth of $10^{6-7}M_{\odot}$ BHs. However, BHs $<10^{6}M_{\odot}$ can grow from TDEs arising from super-Eddington accretion without contributing significantly to the observed AGN LF at $z=0$.

Comments: 10 pages, 6 figures. To be submitted to ApJ

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