230 GHz VLBI observations of M87: event-horizon-scale structure at the enhanced very-high-energy $\rm \gamma$-ray state in 2012

TL;DR

This paper presents 230 GHz VLBI observations of M87 during a VHE gamma-ray flux enhancement, revealing event-horizon-scale structure consistent with jet and accretion disk models, and constraining the VHE emission region size.

Contribution

First measurement of 230 GHz VLBI closure phase on M87, providing new insights into the event-horizon-scale structure during a VHE gamma-ray flare.

Findings

Closure phases are mostly consistent with zero, supporting simple jet and disk models.

Brightness temperature of the core is about 10^10 K, matching lower-frequency core brightness.

VHE gamma-ray emission region is estimated to be 20-60 Schwarzschild radii in size.

Abstract

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 $R_{{\rm s}}$.