3. X-ray catalog study
    4. The accretion process in neutron-star low-mass X-ray binaries

1. Tidal disruption events
When unlucky stars wander too close to a supermassive black hole (SMBH), they can be tidally disrupted and subsequently accreted before they realize this is the end of their world. Such events are called tidal disruption events (TDEs). They provide a unique way to find and study dormant SMBHs believed to reside in the center of most massive galaxies. People have been searching for them for decades, but only about 100 candidates (about 30 in X-rays) were detected thus far.I discovered 6 of them from my intense search of the XMM-Newton and Chandra catalogs, and they all show some unique interesting properties.

(A) Super-Eddington accreting TDEs (For more basic ideas, I refer to my blog.)
Lin, D., Guillochon, J. Komossa, S., Ramirez-Ruiz, E., Irwin, J. A., Maksym, W. P., Grupe, D., Godet, O., Webb, N., A., Barret, D., Zauderer, B., A., Duc P. A., Carrasco, E. R., Gwyn, S. D. J. 2017: “A likely decade-long sustained tidal disruption event,” Nature Astronomy, 1, 0033 (ADS)

The most important achievement that I have in study of TDEs is the discovery of a super-Eddington accreting class of TDEs. It has long been expected that TDEs can easily have peak mass accretion rates well above the Eddington limit for small SMBHs of mass <107 solar mass. But the observational evidence for the super-Eddington accretion in TDEs is very weak. I just discovered a prolonged TDE (XJ1500+0154, Lin et al. 2017, Nature Astronomy), which has remained very luminous for a decade in time and showed only a very modest decay after quickly going into outburst (upper panel in Fig 1.1). This is completely different from other TDEs that typically remain luminous for only a year. Its X-ray spectra are also very unique, generally quasi-soft of characteristic temperatures ~0.3 keV (C2, X3, & C3-C9 observations in Fig 1.1). That is, they were much softer than AGNs, but not as soft as typical thermal TDEs of characteristic temperatures <0.1 keV (dubbed supersoft). More 'crazily', the recent observation by Chandra indicated that the X-ray spectrum has changed the state to be super-soft (C10 in Fig 1.1). Such a dramatic spectral change has never been observed in AGNs or TDEs.

All the four properties (the high luminosity, slow decay, special spectral shapes, and dramatic spectral softening) in concert suggest the presence of a prolonged super-Eddington accretion phase in our event. When the gas is fed/accreted to the SMBH at a not too high rate, the SMBH can "digest it efficiently", the accretion disk maximally converting the gas’ dynamic energy into radiation like X-rays. But when the feeding rate of the gas is too high (super-Eddington accretion), the radiation becomes strong enough to blow away some gas before falling into the SMBH, and some gas will just directly be swallowed by the SMBH without producing light. So the light radiated and observed is self-adjusted to a maximal level called the Eddington luminosity. Therefore the high luminosity and slow decay in our event supports a super-Eddington accretion phase. 

The unique (quasi-soft) spectral shape is also expected for this phase, based on stellar-mass black hole X-ray binaries, in which a small black hole of mass a few times that of the sun accretes mass from an orbiting star. It has been found that the X-ray spectra in the super-Eddington accretion phase in these systems show some special characteristics. The X-ray spectra of our event exhibit similar characteristics, except being about ten thousand times more luminous.

A simple/natural but very intriguing explanation for the spectral softening is that the accretion rate decreased from being super-Eddington to sub-Eddington, a change that has been detected in stellar-mass black hole X-ray binaries. Therefore the spectral softening support the presence of a long super-Eddington accretion phase (at least 6 years) in the event.


Fig 1.1: The temporal evolution of the X-ray luminosity (upper panel) and the X-ray spectra (lower panels) of XJ1500+0154.

XJ1500+0154 is not unique. I have also discovered two more TDE candidates with quasi-soft X-ray spectra, though they were not observed as well as XJ1500+0154. The detailed properties of these two TDEs will be presented in a paper that I am preparing. These three objects thus seem to form the long-sought class of super-Eddington accreting TDEs.

(B) A TDE of strong X-ray quasi-periodic oscillations
Lin, D., Godet, O., Ho, L., C., Barret, D., Webb, N., A., Irwin, J. A. 2017: “Large Decay of X-ray Flux in 2XMM J123103.2+110648: Evidence for a Tidal Disruption Event,” MNRAS, 468, 783 (ADS)
Lin, D., Irwin, J. A., Godet, O., Webb, N. A., Barret, D. 2013: “A ~3.8 hr Periodicity from an Ultrasoft Active Galactic Nucleus Candidate,” ApJL, 776, 10 (ADS)

The X-ray source 2XMM J123103.2+110648 is coincident with a dwarf galaxy whose optical spectrum in fact shows some AGN signature, but the X-ray outburst feature with a large amplitude and pure thermal spectra make the source also consistent with a TDE. There are two interesting properties of the source: one is its low black hole mass of ~105 solar mass, based on narrow emission lines and detailed spectral fits, and the other is the strong ~3.8 hr quasi-periodic X-ray oscillations in the two XMM-Newton observations in December 2005.

Fig 1.2: (left) Long-term bolometric luminosity curve, consistent with a TDE model. (middle) Ultra-soft X-ray spectra in various epochs. (right) Strong ~3.8 hr quasi-periodic oscillations in two XMM-Newton observations in 2005 December.


(C) A TDE of fast-moving warm absorbing outflow
Lin, D., Maksym, W. P., Irwin, J. A., Komossa, S., Webb, N. A., Godet, O., Barret, D., Grupe, D., Gwyn, S. D. J. 2015: “An Ultrasoft X-ray Flare from 3XMM J152130.7+074916: a Tidal Disruption Event Candidate,” ApJ, 811, 43 (ADS)
The X-ray source 3XMM J152130.7+074916 had only one detection showing ultrasoft X-ray spectrum. Its host galaxy shows no AGN activity. Although there is only one X-ray detection, the existence of two deep observations in which the source was not detected allows to constrain the light curve model very well. Compared with other TDEs, this one is interesting because the starting time is well constrained to within months and its X-ray spectrum indicates the presence of a strong fast-moving warm absorbing outflow.
Fig 1.3: (left) the long-term bolometric luminosity curve; (right) spectral fit with a thermal disk subject to a fast-moving warm absorber (upper panel) and the fit residuals without (middle panel) and with (lower panel) the warm absorber.

(D) A TDE of a weak steep hard component
Lin, D., Strader, J., Carrasco, E. R., Godet, O., Grupe, D., Webb, N. A., Barret, D. , Irwin, J. A., 2017: “Multiwavelength Follow-up Observations of the Tidal Disruption Event Candidate 2XMMi~J184725.1-631724,” MNRAS (ADS)
Lin, D., Carrasco, E. R., Grupe, D., Webb, N., Barret, D., Farrell, S. A. 2011: “Discovery of an Ultrasoft X-ray Transient Source in the 2XMM Catalog: A Tidal Disruption Event Candidate,” ApJ, 738, 52 (ADS)
The X-ray source 2XMMi~J184725.1-631724 is the first TDE that I ever discovered. The unique features of this event are the cooling of the disk with the decrease in luminosity, with inner disk radius consistent to be constant, and the presence of a weak steep hard component (Fig. 1.5). To be brief, the disk in this event behaves very similar to the thermal state of the stellar-mass black hole X-ray binary, but there is strong short-term variability consistent with changing inner disk radius at constant accretion rate (thus could be due to some instability).

Fig. 1.4: The luminosity curve.

Fig. 1.5: The spectral fits of the two XMM-Newton X-ray spectra using a multicolor disk plus a power-law.

(E) Off-nuclear TDEs. I also discovered a couple of TDEs associated with two off-nuclear IMBHs. See the IMBH section for more details.