1. Strong lensing of tidal disruption events: Detection rates in imaging surveys led by Kaitlyn Szekerczes (PhD student, Penn state)

Tidal disruption events (TDEs) occur when a star passes sufficiently close to a supermassive black hole (SMBH) (Hills1988, Rees1988). Since quiescent BHs can create TDEs, TDEs can serve as a promising tool for probing these dormant BHs. The population studies of TDEs in dwarf galaxies will provide a better understanding of the BH mass function. Currently, on the order of 100 TDE candidates have been detected with a typical redshift up to 0.2, but this number will exponentially grow to possibly more than thousands with detections by ongoing (e.g., Zwicky Transient Facility and eROSITA) and upcoming surveys (e.g., Rubin Observatory Legacy Survey of Space and Time-LSST and Ultraviolet Transient Astronomy Satellite – ULTRASAT).

Although the detections of TDEs so far have been confined within the nearby Universe, TDEs at comological distances could be detected in the future with observing instruments with deeper fields. For that case, it is possible these events can be gravitationally lensed by galaxies at lower redshifts, magnifying the brightness of the TDEs. Such lensing effects would enable the detections of TDEs at even higher redshifts, which would allow us to study the TDE rates and the BH mass function at the lower mass end and at earlier cosmic time. 

In this work, Kaitlyn Szekerczes (PhD student, Penn State University) led a project with Sherry Suyu, Simon Huber, Jane Dai, Masamuni Oguri, and me where we quantify the rate of both unlensed and lensed TDEs as a function of limiting magnitudes in four different optical bands (u, g, r, and i) by future surveys including LSST, and investigated the lensing properties, using Monte-Carlo simulations.  Dependent on the temperature and luminosity model, we find that g and r bands are the most promising bands with unlensed TDE detections that can be as high as 10,000 annually. The figure below shows the unlensed TDE rates as a function of limiting magnitude for two different luminosity models (L1: accretion-powered and L2: shock-powered). 

In addition, we estimate that a few lensed TDEs can be detected annually in g or r bands in the LSST survey, with TDE redshifts in the range of 0.5 to 2. The ratio of lensed to unlensed detections indicates that we may detect one lensed event for every 10,000 unlensed events. The two figures below show the lensed TDE rates as a function of limiting magnitude for two different luminosity models (L1: accretion-powered and L2: shock-powered). 

The number of lensed TDEs decreases as a function of the image separations and time delays, and most of the lensed TDE systems are expected to have image separations below 3 arc second and time delays within 30 days.
At fainter limiting magnitudes, the i band becomes notably more successful. These results suggest that strongly lensed TDEs are likely to be observed within the coming years and such detections will enable us to study the demographics of black holes at higher redshifts through the lensing magnifications. Check out our paper for more details!