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Unusual Hard X-Ray Flares Caught in NICER Monitoring of the Binary Supermassive Black Hole Candidate AT2019cuk/Tick Tock/SDSS J1430+2303

2023; IOP Publishing; Volume: 945; Issue: 2 Linguagem: Inglês

10.3847/2041-8213/acbea9

2041-8213

Megan Masterson, Erin Kara, Dheeraj R. Pasham, Daniel J. D’Orazio, D. J. Walton, A. C. Fabian, Matteo Lucchini, Ronald A. Remillard, Zaven Arzoumanian, О. Бурхонов, Hyeonho Choi, Sh. A. Ehgamberdiev, E. C. Ferrara, Muryel Guolo, Myungshin Im, Yonggi Kim, D. O. Mirzaqulov, Gregory S. H. Paek, Hyun-Il Sung, Joh-Na Yoon,

Pulsars and Gravitational Waves Research

Abstract 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 2022 January to August, as well as continued optical monitoring during the same time period. We find no evidence of periodic/quasiperiodic modulation in the X-ray, UV, or optical bands; however, we do observe exotic hard X-ray variability that is unusual for typical active galactic nuclei (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 nonflaring data. In its nonflaring 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, multiwavelength monitoring of changing-look AGN like AT2019cuk to probe the corona-jet connection.