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