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Journal Article

The BAT AGN Spectroscopic Survey. XVIII. Searching for supermassive black hole binaries in X-rays


Shimizu,  T. Taro
Infrared and Submillimeter Astronomy, MPI for Extraterrestrial Physics, Max Planck Society;

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Liu, T., Koss, M., Blecha, L., Ricci, C., Trakhtenbrot, B., Mushotzky, R., et al. (2020). The BAT AGN Spectroscopic Survey. XVIII. Searching for supermassive black hole binaries in X-rays. The Astrophysical Journal, 896(2): 122. doi:10.3847/1538-4357/ab952d.

Cite as: https://hdl.handle.net/21.11116/0000-0006-F943-6
Theory predicts that a supermassive black hole binary (SMBHB) could be observed as a luminous active galactic nucleus (AGN) that periodically varies on the order of its orbital timescale. In X-rays, periodic variations could be caused by mechanisms including relativistic Doppler boosting and shocks. Here we present the first systematic search for periodic AGNs using 941 hard X-ray light curves (14–195 keV) from the first 105 months of the Swift Burst Alert Telescope (BAT) survey (2004–2013). We do not find evidence for periodic AGNs in Swift-BAT, including the previously reported SMBHB candidate MCG+11−11−032. We find that the null detection is consistent with the combination of the upper-limit binary population in AGNs in our adopted model, their expected periodic variability amplitudes, and the BAT survey characteristics. We have also investigated the detectability of SMBHBs against normal AGN X-ray variability in the context of the extended ROentgen Survey with an Imaging Telescope Array (eROSITA) survey. Under our assumptions of a binary population and the periodic signals they produce, which have long periods of hundreds of days, up to 13% true periodic binaries can be robustly distinguished from normal variable AGNs with the ideal uniform sampling. However, we demonstrate that realistic eROSITA sampling is likely to be insensitive to long-period binaries because longer observing gaps reduce their detectability. In contrast, large observing gaps do not diminish the prospect of detecting binaries of short, few-day periods, as 19% can be successfully recovered, the vast majority of which can be identified by the first half of the survey.