Parsec-Scale Localization of the Quasar SDSS J1536+0441A, a Candidate
  Binary Black Hole System
  The radio-quiet quasar SDSS J1536+0441A shows two broad-line emission
systems, recently interpreted as a binary black hole (BBH) system with a
subparsec separation; as a double-peaked emitter; or as both types of systems.
The NRAO VLBA was used to search for 8.4 GHz emission from SDSS J1536+0441A,
focusing on the optical localization region for the broad-line emission, of
area 5400 mas^2 (0.15 kpc^2). One source was detected, with a diameter of less
than 1.63 mas (8.5 pc) and a brightness temperature T_b > 1.2 x 10^7 K. New
NRAO VLA photometry at 22.5 GHz, and earlier photometry at 8.5 GHz, gives a
rising spectral slope of alpha = 0.35+/-0.08. The slope implies an optically
thick synchrotron source, with a radius of about 0.04 pc, and thus T_b ~ 5 x
10^10 K. The implied radio-sphere at rest frame 31.2 GHz has a radius of 800
gravitational radii, just below the size of the broad line region in this
object. Observations at higher frequencies can probe whether or not the
radio-sphere is as compact as expected from the coronal framework for the radio
emission of radio-quiet quasars.
