# Discovery of a Luminous Radio Transient 460 pc from the Central Supermassive Black Hole in Cygnus A

## TL;DR

A new luminous radio source near Cygnus A's nucleus suggests a secondary supermassive black hole, potentially influencing the galaxy's jet activity, with evidence from multi-wavelength observations indicating its compactness and luminosity.

## Contribution

This paper reports the discovery of a new radio source near Cygnus A's nucleus, likely a secondary SMBH, providing insights into galaxy dynamics and black hole interactions.

## Key findings

- Radio source flux increased from <0.5 mJy to 4 mJy between 1989 and 2016
- Source is compact down to 4 pc scale in VLBA observations
- Coincides with a near-infrared point source, supporting SMBH interpretation

## Abstract

We report the appearance of a new radio source at a projected offset of 460 pc from the nucleus of Cygnus A. The flux density of the source (which we designate Cygnus A-2) rose from an upper limit of <0.5 mJy in 1989 to 4 mJy in 2016 (nu=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 VLBA 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, although an exotic supernova model cannot be ruled out. The gravitational influence of a secondary SMBH 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 10^7 years.