VLBI imaging of the pre-coalescence SMBHB candidate SDSS
  J143016.05+230344.4
  Context. Recently, SDSS J143016.05+230344.4 (J1430+2303) was reported to be a
supermassive black hole binary (SMBHB) in the final coalescence phase. It is
probably the first SMBHB coalescence event observable in human history. Radio
observations of J1430+2303 before and after coalescence will provide a unique
diagnosis of the energetics and environment of the SMBHB.
  Aims. We explore the radio emission from the galactic nucleus region that is
closely related to the current X-ray and optical activities and helps to
understand the state of black hole accretion and outflow before coalescence.
  Methods. Very long baseline interferometry (VLBI) imaging is the only method
that offers milli-arcsecond-level high resolution that can exclude the
contamination by diffuse emission on galactic scales. We observed J1430+2303
with the European VLBI Network at 1.7 GHz and with the Very Long Baseline Array
at 1.6 and 4.9 GHz in late February and early March 2022.
  Results. A compact component is detected in all three VLBI images. It has a
brightness temperature of > 10^8 K, an unresolved morphology with a size < 0.8
pc, and a flat radio spectrum. These observational features are inconsistent
with large opening-angle outflows or winds, but indicate that this compact
component might be a jet or a corona. Nearly 60% of the emission is resolved by
VLBI and may come from remnant lobes of previous radio activities, the outer
layers of a structured jet, or shocks formed by the disc winds in the narrow
line region.
  Conclusions. Current VLBI images do not yet show signs of radio outbursts.
Our observations provide pre-coalescence radio data that are an important
reference for future comparative studies with the post-merger. In particular,
further resolving the jet will pave the way for probing the dynamical features
associated with inspiralling binary black holes.
