VLBI detection of an AGN pair in the binary black hole candidate SDSS J1536+0441

TL;DR

This study uses VLBI radio imaging to detect two active nuclei in the candidate binary black hole system SDSS J1536+0441, providing direct evidence of a binary black hole pair at parsec scales.

Contribution

First high-resolution VLBI imaging of SDSS J1536+0441 revealing two compact radio cores, confirming the presence of a binary black hole system with active nuclei.

Findings

Detected two compact radio cores at 5 GHz with VLBI.

Both cores have flat or inverted spectral indices.

Radio emission likely powered by thermal free-free emission from disk wind.

Abstract

We present first pc-scale radio imaging of the radio-quiet candidate binary black hole system SDSS J1536+0441. The observations were carried out by the European VLBI Network at the frequency of 5 GHz and allowed to image SDSS J1536+0441 with a resolution of about 10 mas (50 pc). Two compact radio cores are detected at the position of the kpc-scale components VLA-A and VLA-B, proving the presence of two compact active nuclei with radio luminosity about 10^{40} erg/s, thus ruling out the possibility that the two radio sources are both powered by one 0.1 pc binary black hole. From a comparison with published 8.5 GHz flux densities we derived an estimate of the radio spectral index of the two pc-scale cores. Both cores have flat or inverted spectral index and, at least for the case of VLA-A, we can rule out the possibility that synchrotron self-absorption is responsible for the inverted radio spectrum. We suggest that thermal free-free emission from an X-ray heated disk wind may be powering the radio emission in VLA-A.