Exploring the magnetic field configuration in BL Lac using GMVA
  The high radio frequency polarization imaging of non-thermal emission from active galactic nuclei (AGN) is a direct way to probe the magnetic field strength and structure in the immediate vicinity of supermassive black holes (SMBHs) and is crucial in testing the jet-launching scenario. To explore the the magnetic field configuration at the base of jets in blazars, we took advantage of the full polarization capabilities of the Global Millimeter VLBI Array (GMVA). With an angular resolution of $\sim$50 micro-arcseconds ($\mu$as) at 86 GHz, one could resolve scales up to $\sim$450 gravitational radii (for a 10$^9$ solar mass black hole at a redshift of 0.1). We present here the preliminary results of our study on the blazar BL~Lac. Our results suggest that on sub-mas scales the core and the central jet of BL Lac are significantly polarized with two distinct regions of polarized intensity. We also noted a great morphological similarity between the 7mm/3mm VLBI images at very similar angular resolution. 
B. Rani, T. P. Krichbaum, J. Hodgson, S. Koyama, J. A. Zensus, L.   Fuhrmann, A. P. Marscher, S. G. Jorstad
astro-ph.HE