2017ApJ...843...14B

The radio galaxy 0402+379 is believed to host a supermassive black hole binary (SMBHB). The two compact-core sources are separated by a projected distance of 7.3 pc, making it the most (spatially) compact resolved SMBHB known. We present new multi-frequency VLBI observations of 0402+379 at 5, 8, 15, and 22 GHz and combine them with previous observations spanning 12 years. A strong frequency-dependent core shift is evident, which we use to infer magnetic fields near the jet base. After correcting for these shifts we detect significant relative motion of the two cores at β=v/c=0.0054±0.0003 at PA=-34○ / . 4. With some assumptions about the orbit, we use this measurement to constrain the orbital period P~3×10⁴ yr and SMBHB mass M~15×10⁹ M_☉. While additional observations are needed to confirm this motion and obtain a precise orbit, this is apparently the first black hole system resolved as a visual binary.