Discovery of a Radio Transient in Cygnus A

ATel #9495; D. A. Perley (Dark Cosmology Centre, NBI), R. A. Perley, and C. L. Carilli (NRAO)
on 13 Sep 2016; 23:52 UT
Credential Certification: Daniel Perley (dperley@dark-cosmology.dk)

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We report the detection of a new radio source close to the nucleus of Cygnus A. Observations taken with the Very Large Array at frequencies between 8-20 GHz in July 2015, and between 20-50 GHz in August 2016, reveal a point source at the following location (J2000): RA = 19:59:28.32385 Dec = +40:44:01.9165 The source is detected at all frequencies and cleanly resolved from the Cygnus A nucleus; the separation is 0.395" (=430 pc). The positional accuracy (as registered against the nucleus) is approximately 3 mas. The flux density of this source is 4 mJy at 10 GHz, with a spectral index of alpha ~ -0.2 (F_nu ~ nu^alpha). The source shows no evolution (or perhaps a slight brightening) between 2015 and 2016, although the non-overlapping frequency coverage precludes an exact flux comparison. Archival VLA observations of this location taken in 1989 show only the bright (~1 Jy) Cygnus A nucleus and associated jets. We place a limit on the flux density at the transient position of <0.7 mJy (3 sigma) at 8 GHz at that time. The location of the transient is not along the jet axis, but it coincides with a compact optical/near-IR source visible in archival Hubble Space Telescope images and in the K-band adaptive optics imaging published by Canalizo et al (ApJ, 597, 823), previously suggested to be the stripped stellar core of a merging satellite galaxy. The most probable origin of the radio transient is a flare from a previously-unrecognized, secondary active nucleus within the Cygnus A host galaxy. However, other interpretations (including a very luminous supernova or the tidal disruption of a star by a secondary SMBH) are also consistent with the available data. Additional radio observations are being pursued. The close proximity to the Cygnus A nucleus makes follow-up of this source challenging or impossible with most telescopes, but additional observations at high-resolution facilities are encouraged to help constrain the nature of this source.