The BAT AGN Spectroscopic Survey. XVIII. Searching for Supermassive Black Hole Binaries in X-Rays

Tingting Liu, Michael Koss, Laura Blecha, Claudio Ricci, Benny Trakhtenbrot, Richard Mushotzky, Fiona Harrison, Kohei Ichikawa, Darshan Kakkad, Kyuseok Oh, Meredith Powell, George C. Privon, Kevin Schawinski, T. Taro Shimizu, Krista Lynne Smith, Daniel Stern, Ezequiel Treister, C. Megan Urry

Research output: Contribution to journalArticlepeer-review


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.

Original languageEnglish
Article number122
JournalAstrophysical Journal
Issue number2
StatePublished - 20 Jun 2020


FundersFunder number
National Science Foundation
Directorate for Mathematical and Physical Sciences1715413, 1430284


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