VLBI Detection of an Active Galactic Nucleus Pair in the Binary Black Hole Candidate SDSS J1536+0441

We present the 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 a frequency of 5 GHz, allowing the imaging of SDSS J1536+0441 with a resolution of \~{}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 L$_{R}$ \~{} 10$^{40}$ erg s$^{-1}$, thus ruling out the possibility that both radio sources are powered by a 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 a flat or an 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.