Implications from Late-time X-Ray Detections of Optically Selected Tidal Disruption Events: State Changes, Unification, and Detection Rates

Abstract

We present Chandra X-ray observations of four optically-selected tidal disruption events (TDEs) obtained 4-9 years after their discovery. Three sources were detected with luminosities between 9×10 40 and 3 × 10 42 erg s −1 . The spectrum of PTF09axc is consistent with a power law with index of 2.5±0.1, whereas the spectrum of PTF09ge is consistent with the Wien tail of a soft black body best described over the 0.3-7 keV range with a power law of index 3.9±0.5 (the best-fit black body temperature is 0.18 ± 0.02 keV). The power law spectrum of PTF09axc may signal that TDEs transition from an early-time soft state to a late-time low-hard state many years after disruption. The mismatch in Eddington fractions of these sources (≈ 5% for PTF09axc; ≈ 0.2% for PTF09ge) could indicate that, as is the case for X-ray binaries, mass accretion rate is not the sole parameter responsible for TDE state changes. These detections can be used to shed light on the difference between optically selected vs. X-ray selected TDEs. We propose that the time to peak luminosity for optical and X-ray emission may differ substantially in an individual TDE, with X-rays being produced or becoming observable later. This delay can serve to explain the differences in observed properties such as L opt /L X of optically and X-ray selected TDEs. Using our observations to calibrate simple models for TDE X-ray light curves, we update predictions for the soon-to-be-launched eROSITA instrument, finding an eROSITA TDE detection rate of 3 yr −1 to 990 yr −1 , a range that depends sensitively on (i) the distribution of black hole spins and (ii) the typical time delay between disruption and peak X-ray brightness. We further predict an asymmetry in the number of retrograde and prograde disks in samples of optically and X-ray selected TDEs, even if the intrinsic number of stars on pro-and retrograde TDE orbits is equal. X-ray selected TDE samples will have a strong bias towards prograde disks (up to 1-2 orders of magnitude if most supermassive black holes spin rapidly, and less so if most spin slowly). On the other hand, in flux-limited samples of optically-selected TDEs, there seems to exist a more modest (typically factor of a few) bias for either retrograde or prograde disks, depending on the underlying optical emission mechanism and regime of loss cone repopulation. These observational biases can contribute to observed differences between optically and X-ray selected TDEs (with optically selected TDEs being fainter in X-rays if the TDE disk is retrograde).

X-Ray Properties of TDEs

“…In RBS 1032 the spectrum after 20 years had hardened from Γ = 5 to Γ = 3.4 (Maksym et al 2014a). Interestingly, Jonker et al (2019) found late-time X-ray emission in three optically-selected TDEs. The spectra of these could be modeled with power-laws of Γ = 2.5−3.9, compatible with those of the X-ray selected events.…”

“…For reasons which are currently not well understood, optically and UV-bright TDEs tend to show little or no X-ray emission (Gezari et al 2009;Arcavi et al 2014;Jonker et al 2019, and see the Optical Chapter). Nevertheless there are some notable exceptions, two of which are examined below.…”

“…This diversity in the observational characteristics of these events has been suggested to be a natural result of the viewing-angle with respect to the orientation of the accretion disk (e.g., , or that X-ray TDEs and optical/UV only TDEs could result from a separate class of events that have compact debris disk due to large apsidal precession of the self-intersecting streams of the disrupted star (e.g., Dai et al 2015). Jonker et al (2019) discuss three optically-selected TDEs which were not detected in X-rays during the optical flare but 8-10 years later had luminosities of L X ∼ 10 41−42 . In the reprocessing model, the X-ray emission from these events will have been massively suppressed at early times and only become visible when the density of the absorbing material decreased.…”

Tidal Disruptions of White Dwarfs: Theoretical Models and Observational Prospects

“…The X-ray spectral shape of these fast events is consistent with a power law (see the discussion and references in the contribution of Saxton et al to this Volume). This is in contrast to the early-time soft X-ray spectra typically found in TDEs involving more massive black holes and stars other than white dwarfs (for some references and a discussion on differences between earlyand late-time X-ray spectra of TDEs see for instance Jonker et al 2019). It seems unlikely that a difference in the (Eddington ratio) mass accretion rate in white dwarf and main-sequence TDEs is the main cause of the different spectral shape 2 .…”

“…Since power-law X-ray spectra are thought to be caused by Compton up-scattering of seed disc photons by a coronal electron population, which in turn is thought to be accelerated to relativistic energies in magnetic reconnection events (Merloni and Fabian 2001), a power-law spectrum may more naturally emerge in the TDE discs with large magnetisation such as those occurring in white dwarf disruptions. Note that a main-sequence/longer duration TDEs with hard power law like spectra are also more difficult to distinguish from AGN activity and therefore observers may be biased against classifying such events as TDEs (Jonker et al 2019).…”