VLBI imaging of the pre-coalescence SMBHB candidate SDSS J143016.05+230344.4

¹ SKA Regional Centre Joint Lab, Shanghai Astronomical Observatory, CAS, Nandan Road 80, Shanghai 200030, PR China
e-mail: antao@shao.ac.cn
² SKA Regional Centre Joint Lab, Peng Cheng Lab, Shenzhen 518066, PR China
³ College of Astronomy and Space Sciences, University of Chinese Academy of Sciences, 19A Yuquanlu, Beijing 100049, PR China
⁴ Department of Physics, Anhui Normal University, Wuhu, Anhui 241002, PR China
⁵ Perimeter Institute for Theoretical Physics, Waterloo, ON N2L2Y5, Canada
⁶ University of Guelph, Department of Physics, Guelph, ON N1G2W1, Canada
⁷ CAS Key Laboratory for Research in Galaxies and Cosmology, Department of Astronomy, University of Science and Technology of China, Hefei, Anhui 230026, PR China
⁸ School of Astronomy and Space Science, University of Science and Technology of China, Hefei 230026, PR China
⁹ Department of Astronomy, Guangzhou University, Guangzhou 510006, PR China
¹⁰ Key Laboratory for Research in Galaxies and Cosmology, Shanghai Astronomical Observatory, CAS, Nandan Road 80, Shanghai 200030, PR China

Received: 20 April 2022
Accepted: 3 May 2022

Abstract

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 > 10⁸ 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.