A Binary Supermassive Black Hole

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D.A. Perley et al. report the serendipitous discovery of a new radio-wavelength source at a projected offset of 460 parsecs (pc) from the nucleus of Cygnus A, one of the best-studied powerful radio galaxies and the archetype of a class II radio galaxy, in which two powerful oppositely directed jets of relativistic matter are observed to emanate from a central point source at the galaxy nucleus and terminate at bright hotspots in extensive edge-brightened radio lobes in the halo.

The flux density of the new source – designated Cygnus A-2 – rose from an upper limit of < 0.5 milli-Jansky in 1989 to 4 milli-Jansky in 2016 (ν = 8.5 GHz), but is currently not varying by more than a few percent per year. The radio luminosity of the source is comparable to the most luminous known supernovae, it is compact in Very Long Baseline Array observations down to a scale of 4 pc, and it is coincident with a near-infrared point source seen in pre-existing adaptive optics and HST observations. The most likely interpretation of this source is that it represents a secondary supermassive black hole in a close orbit around the Cygnus A primary, though an exotic supernova model cannot be ruled out. The gravitational influence of a secondary supermassive black hole at this location may have played an important role in triggering the rapid accretion that has powered the Cygnus A radio jet over the past 107 years.

Image: [Left] A wide-field image of the Cygnus A region. The background is Gemini Observatory optical imaging; the contours are VLA data at 2 GHz from 2015 November, showing the iconic jet and lobe structure. [Right] A zoom into the Cygnus A nuclear region. The background is  Keck Observatory Adaptive Optics imaging from Canalizo et al. (2003). The radio contours are from a 35 GHz VLA image acquired in 2016 October. A distinct, luminous point source is detected 0.42 arcsec from the luminous nucleus in the radio and in the near infrared imaging.

Publication: D. A. Perley (Liverpool John Moores University, University of Copenhagen) et al., Discovery of a Luminous Radio Transient 460 pc from the Central Supermassive Black Hole in Cygnus A, 2017, Astrophysical Journal, 841, 117.

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