Broadband Multi-wavelength Properties of M87 during the 2017 Event Horizon Telescope Campaign

TL;DR

This paper presents the first comprehensive multi-wavelength spectrum of M87 during the 2017 EHT campaign, revealing insights into its jet structure and emission mechanisms through combined high-resolution and broad-spectrum data.

Contribution

It provides the most complete simultaneous multi-wavelength spectrum of M87's core, analyzes data from diverse sources, and applies models to understand jet properties and emission regions.

Findings

M87 was in a historically low flux state during the campaign.

Core flux dominated over HST-1 at high energies, enabling combined analysis.

Gamma-ray emission likely originates from inner jets, not the same region as EHT mm-band emission.

Abstract

In 2017, the Event Horizon Telescope (EHT) Collaboration succeeded in capturing the first direct image of the center of the M87 galaxy. The asymmetric ring morphology and size are consistent with theoretical expectations for a weakly accreting supermassive black hole of mass approximately 6.5 x 10^9 M_solar. The EHTC also partnered with several international facilities in space and on the ground, to arrange an extensive, quasi-simultaneous multi-wavelength campaign. This Letter presents the results and analysis of this campaign, as well as the multi-wavelength data as a legacy data repository. We captured M87 in a historically low state, and the core flux dominates over HST-1 at high energies, making it possible to combine core flux constraints with the more spatially precise very long baseline interferometry data. We present the most complete simultaneous multi-wavelength spectrum of the active nucleus to date, and discuss the complexity and caveats of combining data from different spatial scales into one broadband spectrum. We apply two heuristic, isotropic leptonic single-zone models to provide insight into the basic source properties, but conclude that a structured jet is necessary to explain M87's spectrum. We can exclude that the simultaneous gamma-ray emission is produced via inverse Compton emission in the same region producing the EHT mm-band emission, and further conclude that the gamma-rays can only be produced in the inner jets (inward of HST-1) if there are strongly particle-dominated regions. Direct synchrotron emission from accelerated protons and secondaries cannot yet be excluded.