Hard X-ray Flares in NICER Monitoring of Binary SMBH Candidate AT2019cuk

The nuclear transient AT2019cuk/SDSSJ1430/“Tick Tock source” has been suggested to harbor a supermassive black hole (SMBH) binary within a few years of coalescence (Jiang et al. 2022). Since January 2022, we have been monitoring this source with high-cadence NICER X-ray observations (multiple visits per day) and continued optical monitoring. I will present the results of this monitoring campaign, which shows no obvious periodic/quasi-periodic modulation on the reported timescale of ~30 days in either the X-ray or the optical bands. However, the NICER X-ray data reveal repetitive hard X-ray flares (in 2-4 keV) that have never been seen before in other AGN. These X-ray flares last for roughly a day and exhibit anomalously hard X-ray spectra. Regardless of the model chosen, the flares have low photon indices of ~1-1.4, which is much harder than what is seen in standard AGN and in the non-flaring NICER data. Thus, whether a binary SMBH or not, AT2019cuk is undoubtedly an exotic nuclear transient with unique X-ray variability. In this talk, I will present and discuss potential models to explain these hard flares, including: (1) a variable obscuration model, (2) corona/jet variability, and (3) if this is indeed a binary, self-lensing from the secondary SMBH. However, the self-lensing model would predict strictly periodic X-ray flares, which does not seem to be consistent with current data.