1. Dongyue Li
15 July 2019

2. Abstract
Methods. Chandra X-ray observations of four optically-selected TDEs obtained 4-9 years after the discovery. Spectral analysis was performed. To calibrate the simple models of X-ray light curves and update predictions for the soon-to-be-launched eROSITA instrument (dependence).
Results.
(1) the mismatch in the Eddington fraction of these sources could indicate that as is the case for XRB, mass accretion rate is not the sole parameter responsible for TDE state changes.
(2) Propose that the time to peak luminosity for optical and X-ray emission may differ substantially in an individual TDE, with X-ray being produced later.
(3) Predict an asymmetry in the number of retrograde and prograde disks in the samples of optically and X-ray selected TDEs. X-ray selected TDE samples will have a strong bias towards prograde disks.

3. Background Challenges: (discovery of optically selected TDEs):
i. Low BB temperature, large bb radii, ii. Little cooling over a ~100 day baseline, iii. A steep power law decay, iiii. Very high opt/UV luminosity
Tidal disruption events (TDEs)
Theoretical alternatives for the observed opt/UV emission:
photon-driven or line-driven outflows
Emission powered by shocks
Thermal reprocessing of accretion power by a layer of gas at large radii
Soft X-ray observations of TDEs:
Very soft spectra, BB temperature < 0.1 keV
Many optically selected TDEs go undetected in X-ray and, vise versa
Early model for TDEs: compact accretion disk, quick circularization, mostly in UV and soft X-ray
Circularization might be very slow
Note: the X-ray spectra are always taken in the first one or two years, when the accretion rate is high

4. Background
As TDE accretion rates decline from super-Eddington levels, to modestly sub-Edd, to very sub-Edd levels, their accretion disks might exibite state changes analogous to those of stellar BHXRBs.
Two differences between the early- and late-time phases:
The large uncertainties in the circularization and disk formation will be less important in the late-time phase
TDE disks may pass through the range of sub-Edd accretion rates that produces a state change in XRB disks.
Further motivation:
Detection of late-time FUV emission in optically-selected TDEs.
Slower rate of decline: a transition from fallback-dominated to disk-dominated accretion rates
Small fitted radii: if optically thick reprocessing layers once existed, they have since dissipated

5. Observations, analysis, results
Chandra observations

6. Observations, analysis, results
PTF09axc: best fit as a comparatively hard, non-thermal power law.
PTF09ge: best fit as the Wien tail of a thermal blackbody spectrum.

7. Discussion ASASSN-14ae: 4.8 years after the its discovery
Other sources are observed 8~10 yrs after discovery
Our Chandra detections are consistent with 2014 detections and upper limits
FUV detections of these sources were
used to produce disk models and estimates for a range
of quasi-thermal soft X-ray luminosities; the range of
modeled X-ray predictions is particularly sensitive to
the dimensionless SMBH spin parameter, . While
our detection of ASASSN-14ae is compatible with the
lower end of the predicted range.
Our detection of PTF09ge is considerably brighter than the predicted range. This discrepancy could be reconciled by invoking even larger values of prograde SMBH spin and/or
a SMBH mass somewhat smaller than the fiducial prediction.
PTF09axc was not observed at late times in FUV
Non-detection of PTF09djl is compatible with prediction.

8. Disk state changes Less observed in AGN For TDEs, at late time:
a factor of times larger than the
typical TDE disk.
When a source in a high-soft state experiences
a persistent decline in accretion rate, it will typically transition to a low-hard state once m_ falls below a threshold value 0.03.

9. Disk state changes
Early-time soft X-ray detections of TDE candidates generally
find quasi-thermal spectra that are analogous to
a high-soft state.
The best-studied TDE X-ray
spectra are qualitatively closer to an XRB high{soft state
than they are to AGN power laws.: (1) the smaller SMBH mass, (2) Higher Eddington fraction at early time, (3) a bias towards prograde spinning SMBH in X-ray selected TDEs
Observationally, TDE candidates with soft spectra
containing an additional hard, power-law X-ray spectral
components do exist, much like XRB soft states
where a sub-dominant power-law component also exists.
Our results: PTF09axc has undergone a state
change to the SMBH analogue of the low-hard state,
but this type of change has not yet occurred for PTF09ge,
which likely remains in an analogue of the high-soft state.
The absence of super-Eddington emission may be related to a delay before sources are detected in X-ray
The X-ray spectrum in the TDE candidate RX J
changed from a power-law with gamma~ 5 (so a very soft
spectrum that could also be t with a blackbody with a
temperature of 0.06 keV) to gamma~2.5 at late times.

10. Disk state changes PTF09axc: PTF09ge: ASASSN_14ae:
However, we note that in XRBs, the emergence of a coronal power-law and the ensuing state change is regulated not only by the accretion rate m_ but also by an additional parameter.
For example, the relatively weak magnetic fields of main sequence stars may mean that TDE disks are born with extremely low magnetizations.
PTF09axc:
PTF09ge:
ASASSN_14ae:

11. Optical vs. X-ray selected TDEs
Do these transients all really stem from the
same underlying type of event?
A different, possibly complementary, way to unify
TDE candidates across a broad range of LX /Lopt ratios
is to postulate a strong temporal, rather than angular,
dependence in LX/Lopt.
If the optical emission is caused by circularization shocks,
a delay between optical and X-ray would be related to
delays in forming the (inner, X-ray emitting) accretion
disk.
If the optical is instead caused by reprocessing of the inner
disk's X{rays and EUV, then an enshrouded inner
disk will only become visible in X-rays after the reprocessing
screen has diluted enough to permit an ionization
Breakout.
viewing angle dependent unified model. Dai et al. 2018

12. Optical vs. X-ray selected TDEs
Because the low LX values we observe are compatible
with past X-ray non-detections (or, in the case of
PTF09axc, its 2014 detection), we are unable to say
whether this truly represents brightening of initially X-
ray dim TDEs. However, deep limits on the X-ray luminosity
in several other optically selected TDEs suggests
that brightening is certainly plausible.
The offset between the peaks of optical and X-ray emission is a key parameter for testing the idea of unification in time, rather than (or in addition to) angle.
The distributions of delay will depend on the emission mechanism for the optical and X-ray light, as well as on event parameters such as beta, black hole mass, and spin.
This scenario also implies that all optically selected
TDEs will at some point emit X{ray radiation,
Sources detected both in optical and X-ray at early times:
(1) efficient circularization

13. Conclusions
Chandra X-ray observations of four optically{selected TDEs long after the peak of their optical flares. In three cases, we detected the late-time soft X-ray emission.
These TDEs exhibit a diversity of X-ray behavior at late times: (1) PTF09ge: best fit as the Wien tail of a thermal blackbody spectrum. (2) PTF09axc: best fit as a comparatively hard, non-thermal power law.
Late time X-ray detections are further evidence that these source represent bonafide TDEs.
The marked spectral differences between PTF09axc and PTF09ge may have been caused by late-time state change in PTF09axc to a low hard state.
Most TDEs are persistently bright X-ray sources visible for at least a decade, which has implications for detection rates in near future, wide field X-ray surveys.
There is often a delay between the peak optical and X-ray emission in TDEs