NuSTAR Observations of Candidate Subparsec Binary Supermassive Black Holes

We present analysis of NuSTAR observations of three active galactic nuclei (AGN) that were identified as candidate subparsec binary supermassive black hole (SMBH) systems in the Catalina Real-Time Transient Survey based on the apparent periodicity in their optical light curves. Many simulations predict that close-separation accreting SMBH binaries will have different X-ray emissions than single accreting SMBHs, including harder spectra and greater luminosity in the X-ray band. Previously we had obtained Chandra observations of these AGN and found no clear differences between their X-ray emissions and the larger AGN population. However, some models predicted the differences to show up in the hard X-rays. We therefore obtained NuSTAR observations (analyzed alongside simultaneous Swift or XMM-Newton observations when possible) to see if these differences were present in the hard X-rays. We find no evidence that the X-ray emissions of these AGN are different from the larger AGN population over the energies detected by NuSTAR. We can therefore rule out the hypothesis that the reason for the lack of difference seen in the Chandra spectra are due to the differences being found in harder X-rays, as was predicted by some models.