Over the past week, NuSTAR observed 4C +37.11, the only confirmed supermassive black hole binary in an active galaxy with spatially resolved orbital motion. This system was originally identified in 2004 by the Very Long Baseline Array (VLBA) as a pair of central, compact, flat-spectrum, variable radio sources with a measured separation of 7.3 pc—evidence of a pair of gravitationally bound supermassive black holes. Subsequent VLBA observations claim the detection of relative proper motion between the binary components. Numerical simulations predict that accretion in supermassive black hole binaries can generate excess hard X-ray emission above 20 keV through shocks and heated mini-disks fed by circumbinary gas streams. NuSTAR will test those predictions of a significant hard excess, or see if the X-ray spectrum follows a standard AGN power law. This test of binary accretion physics in a wide-separation supermassive black hole binary will establish whether a hard X-ray diagnostic that can be applied to unresolved binary AGN candidates identified by LSST and future time-domain surveys.

Author: Daniel Stern (NuSTAR Deputy Principal Investigator, Caltech)