230 GHz VLBI observations of M87: event-horizon-scale structure at the
  enhanced very-high-energy $\rm \gamma$-ray state in 2012
  We report on 230 GHz (1.3 mm) VLBI observations of M87 with the Event Horizon
Telescope using antennas on Mauna Kea in Hawaii, Mt. Graham in Arizona and
Cedar Flat in California. For the first time, we have acquired 230 GHz VLBI
interferometric phase information on M87 through measurement of closure phase
on the triangle of long baselines. Most of the measured closure phases are
consistent with 0$^{\circ}$ as expected by physically-motivated models for 230
GHz structure such as jet models and accretion disk models. The brightness
temperature of the event-horizon-scale structure is $\sim 1 \times 10^{10}$ K
derived from the compact flux density of $\sim 1$ Jy and the angular size of
$\sim 40 $ $\rm \mu$as $\sim$ 5.5 $R_{{\rm s}}$, which is broadly consistent
with the peak brightness of the radio cores at 1-86 GHz located within $\sim
10^2$ $R_{{\rm s}}$. Our observations occurred in the middle of an enhancement
in very-high-energy (VHE) $\rm \gamma$-ray flux, presumably originating in the
vicinity of the central black hole. Our measurements, combined with results of
multi-wavelength observations, favor a scenario in which the VHE region has an
extended size of $\sim$20-60 $R_{{\rm s}}$.
