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

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 $R_{{\rm s}}$.

230 GHz VLBI Observations of M87: Event-Horizon-Scale Structure and Enhanced VHE Gamma-Ray State in 2012

The paper titled "230 GHz VLBI observations of M87: event-horizon-scale structure at the enhanced very-high-energy $\gamma$-ray state in 2012" presents advanced interferometric observations of the radio galaxy M87 using the Event Horizon Telescope (EHT). The research reported first VLBI closure phase measurements at 230 GHz, allowing for a more detailed exploration of the event-horizon-scale structures thought to originate near the central supermassive black hole.

Observations and Methods

The study employed VLBI techniques at 230 GHz (1.3 mm), focusing on M87 during an episode of enhanced very-high-energy (VHE) $\gamma$-ray emission detected in March 2012. The array consisted of telescopes situated at Mauna Kea, Mount Graham, and Cedar Flat. Closure phases on long-baseline triangles determine structural properties beyond mere amplitude measurements, making possible an investigation into the asymmetry expected from theoretical models of M87.

The observations rely on sophisticated calibration techniques to correct for phase fluctuations induced by atmospheric conditions and hardware limitations. This step was critical due to the fast atmospheric changes and the limited number of available millimeter-wavelength stations.

Results

Key observations indicate the presence of an event-horizon-scale structure with a brightness temperature around $1 \times 10^{10}$ Kelvin. This compact emission region exhibited a flux density of approximately 1 Jy and an angular size of about 40 microarcseconds, equating to roughly 5.5 Schwarzschild radii ($R_s$).

The closure phases measured closely adhered to zero degrees, aligning well with predictions from jet and accretion disk models. These findings uphold the idea that the emission originates from either synchrotron processes within the jet structure or the region adjacent to the accretion disk.

During the observation period, M87 was undergoing a VHE $\gamma$-ray flare, potentially linked to dynamics near the black hole. The implicated size of the VHE emission region was constrained to 20-60 $R_s$. The interference data suggests that while a higher flux density was noted across arcsecond scales, no significant structural change was apparent on horizon scales compared to observations from 2009.

Implications and Future Directions

The results support extended source models during VHE enhancements, challenging compact emission theories. These findings readily merge with one-zone synchrotron self-Compton models. However, conflicting with extremely compact regions composed of alternative particle acceleration mechanisms proposed in multiple hadronic and leptonic frameworks.

The practical implications extend to better understanding the magnetic field distribution in areas adjacent to the black hole and apply constraints on radio and VHE emission mechanisms. The study demonstrates that future observations incorporating more VLBI stations, such as the Large Millimeter Telescope and the Atacama Large Millimeter/submillimeter Array, could refine these models. This will enhance testing of theoretical models predicting gravitational lensing and relativistic jet formation in strong-field settings.

Ultimately, such work pushes the boundary of current astronomical techniques, integrating millimeter VLBI observations with multi-wavelength data to unveil phenomena near supermassive black holes. Enhanced resolution and sensitivity from forthcoming EHT developments will likely yield critical insights into relativistic jet formation, magnetic field dominance, and high-energy particle acceleration in astrophysical contexts.