Kavli Affiliate: Dheeraj R. Pasham
| First 5 Authors: Megan Masterson, Erin Kara, Dheeraj R. Pasham, Daniel J. D’Orazio, Dominic J. Walton
| Summary:
The nuclear transient AT2019cuk/Tick Tock/SDSS J1430+2303 has been suggested
to harbor a supermassive black hole (SMBH) binary near coalescence. We report
results from high-cadence NICER X-ray monitoring with multiple visits per day
from January-August 2022, as well as continued optical monitoring during the
same time period. We find no evidence of periodic/quasi-periodic modulation in
the X-ray, UV, or optical bands, however we do observe exotic hard X-ray
variability that is unusual for a typical AGN. The most striking feature of the
NICER light curve is repetitive hard (2-4 keV) X-ray flares that result in
distinctly harder X-ray spectra compared to the non-flaring data. In its
non-flaring state, AT2019cuk looks like a relatively standard AGN, but it
presents the first case of day-long, hard X-ray flares in a changing-look AGN.
We consider a few different models for the driving mechanism of these hard
X-ray flares, including: (1) corona/jet variability driven by increased
magnetic activity, (2) variable obscuration, and (3) self-lensing from the
potential secondary SMBH. We prefer the variable corona model, as the
obscuration model requires rather contrived timescales and the self-lensing
model is difficult to reconcile with a lack of clear periodicity in the flares.
These findings illustrate how important high-cadence X-ray monitoring is to our
understanding of the rapid variability of the X-ray corona and necessitate
further high-cadence, multi-wavelength monitoring of changing-look AGN like
AT2019cuk to probe the corona-jet connection.
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