AT2019cuk/SDSSJ1430/ZTF18aarippg: High-cadence NICER and NuSTAR X-ray observations of the potential supermassive black hole binary with imminent merger (the tick-tock source)

ATel #15225; Dheeraj Pasham (MIT), Andrew Fabian (U. Cambridge), Dom Walton (U. Hertfordshire), Keith Gendreau (NASA/GSFC), Zaven Arzoumanian (NASA/GSFC), Megan Masterson (MIT), Erin Kara (MIT), Ron Remillard (MIT)
on 17 Feb 2022; 03:59 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Dheeraj Pasham (drreddy@mit.edu)

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The nuclear optical transient AT2019cuk/SDSSJ1430+2303/ZTF18aarippg was identified by ZTF on 30 March 2019. Based on its optical modulations with apparently decreasing period and unusual optical emission line shifts over the last 2.5 years, it has been suggested to be a supermassive black hole (SMBH) binary that could merge within the next three years (Jiang et al. 2022). While the probability of finding such a system within <1 Gpc is very low, the scientific potential of studying a bonafide SMBH binary close to coalescence is large.

NICER has been monitoring AT2019cuk since 19 Jan 2022 on a daily basis and has carried out multiple visits over a few days to coordinate with a 114 ks NuSTAR exposure. NICER is detecting the source in the 0.3-4.0 keV with background-subtracted count rates between 1.7 and 3.5 counts/sec. NuSTAR's 3-70 keV count rate was 0.07 cps with no obvious variability over the 220 ks observation period.

The NICER light curve so far has covered a baseline of 24 days and has a modulation roughly consistent with the ~30 d Swift/XRT modulation (Jiang et al. 2022). The overall flux gradually increased by a factor of 2, peaked at roughly 30 d since the last maximum seen in Swift/XRT data, and dropped back in the last week (see https://space.mit.edu/~dheeraj/cuk.html). Data over a much longer baseline will be needed to validate this potential period, since red noise variations can easily produce such apparently periodic modulations.

X-ray spectral fits for an accretion disk with reflection can provide geometrical information about the observer's line of sight, which may vary systematically with time for this system. We fit the 7-70 keV NuSTAR spectrum with a power law modified by Galactic absorption (2.3e20 cm^-2). Extrapolating this model over the entire 0.3-70 keV band (0.3-4 keV NICER and 3-70 keV NuSTAR) shows several features in the data/model plot: 1) systematic residuals between 0.3-1 keV, 2) emission line reminiscent of a relativistically broadened Fe line between 4.5-7 keV, and 3) a Compton hump-like feature above 10 keV. We then jointly fit the combined spectrum with a model consisting of X-ray reflection (using relxill; Garcia et al. 2014) plus a more distant, partially covering warm absorber (using XSTAR; Kallman & Bautista 2001). This model gives a good fit (joint chi-square of 303/347 DoF), reflection is preferred, although is relatively weak, with a reflection fraction of R = 0.19(+0.12 -0.07); 90% uncertainties. Fits without reflection (i.e., AGN continuum and warm absorber) are worse by delta-chisq = 32 for 4 fewer DoF. The reflection is highly ionized, with log[xi] = 3.0(+0.2 -0.1), suggesting an accretion disk origin. We find an inner radius of Rin = 7+/-3 Rg and an inclination of i < 21 deg (assuming a fixed powerlaw emissivity with q=3). The ionized absorption has a column density and ionization parameter of Nh = 2.6(+0.8,-0.9)e22 cm^-2 and log[xi/(erg cm/s)]=1.56(+0.16 -0.26), fairly typical of AGN warm absorbers (e.g., Laha et al. 2014). The covering factor is Cf = 0.44 +/- 0.04. The intrinsic continuum also appears to be fairly typical, with Gamma = 1.76(+0.03 -0.10) and Ecut > 140 keV (Ricci et al. 2017).

NICER will continue monitoring AT2019cuk on a daily basis for the next several months as part of a GO program. We encourage follow-up observations of this potentially very exciting target. A weekly updated schedule can be found here: https://heasarc.gsfc.nasa.gov/docs/nicer/schedule/nicer_sts_current.html. To coordinate multiwavelength observations please contact drreddy@mit.edu

NICER carries out prompt follow-up observations of X-ray-bright extragalactic transients and tracks alerts from LIGO/VIRGO and other facilities. NICER is a 0.2-12 keV X-ray telescope operating on the International Space Station. The NICER mission and portions of the NICER science team activities are funded by NASA. Jiang, N., Yang, H., Wang, T., et al. 2022, arXiv:2201.11633