A Comparison of the X-Ray Emission from Tidal Disruption Events with those of Active Galactic Nuclei

Abstract

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 a TDE. Here we compare the X-ray properties of TDEs and 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 AGN found at similar redshifts. However, compared to preflare upperlimits, highly variable AGN can produce flares of a similar order of magnitude as those seen from X-ray TDEs. Nevertheless, TDEs decay significantly more monotonically, and their emission exhibits little 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 redshifts. We derive the X-ray luminosity function (LF) for X-ray TDEs using the events from Auchettl et al. (2017). Interestingly, our X-ray LF matches closely the theoretically derived LF by Milosavljević et al. (2006) which assumes a higher TDE rate currently estimated from observations. Using our results and the results of Stone & Metzger (2016), we estimate a TDE rate of (0.7 − 4.7) × 10 −4 yr −1 per galaxy, higher than current observational estimates. We find that TDEs can contribute significantly to the LF of AGN for z 0.4, while there is no evidence that TDEs influence the growth of 10 6−7 M BHs. However, BHs < 10 6 M can grow from TDEs arising from super-Eddington accretion without contributing significantly to the observed AGN LF at z = 0.

To TDE or not to TDE: the luminous transient ASASSN-18jd with TDE-like and AGN-like qualities

“…The X-ray spectra of AGNs share similar properties with ASASSN-18jd, as AGNs often show a combination of a kT ∼ 100 eV blackbody and a flat power law with Γ 2 (Ricci et al 2017;Auchettl et al 2018). Additionally, the variable hardness ratio and the soft X-ray flare near +140 d are behaviors that are more commonly seen in AGNs than in TDEs .…”

PS16dtm: A Tidal Disruption Event in a Narrow-line Seyfert 1 Galaxy

“…If for the moment we adopt the simple assumption that the flare rate is equal in galaxies of all types, this suggests that approximately 10% of disruptions should occur in galaxies with L 0.01 L Edd . With the number of TDE candidates currently numbering in the dozens (Auchettl et al 2016), the possibility of a tidal disruption having occurred in one or more nearby active galaxies seems likely. Disruptions about non-accreting black holes with no preexisting disk may suffer from a "dark year" problem where the onset of accretion may be significantly delayed and prolonged, smearing a flare that would usually rise over a period of a month to timescales of up to a decade (Guillochon & Ramirez-Ruiz 2015).…”

A Dependence of the Tidal Disruption Event Rate on Global Stellar Surface Mass Density and Stellar Velocity Dispersion

“…Theoretical formulations predict a TDE rate of~--10 yr 4 1 per galaxy (Magorrian & Tremaine 1999;Wang & Merritt 2004;Stone & Metzger 2016). While some observational studies have estimated similar rates (e.g., Esquej et al 2008;Auchettl et al 2018), others imply a rate an order of magnitude lower (e.g., Donley et al 2002;Gezari et al 2008;van Velzen & Farrar 2014;Holoien et al 2016b).…”

“…11 However, it is still unclear what observational features define a TDE as opposed to, for example, a flaring active galactic nucleus (AGN) or a supernova (see, e.g., Auchettl et al 2018; or the discussion surrounding ASASSN15lh, e.g., Dong et al 2016;Leloudas et al 2016;Liu et al 2017;Margutti et al 2017). Moreover, most objects do not have multiwavelength coverage, and it is unclear if the objects we collectively call "TDE candidates" all belong to one monolithic class.…”