In 2017 the Event Horizon Telescope (EHT) Collaboration captured the first image of the black hole in the centre of the M 87 galaxy. It showed a ring morphology and a size consistent with a supermassive black hole with a mass of ~6.5 billion solar masses. In parallel to the EHT measurements, the EHT Multiwavelength WG organised an extensive multi-wavelength campaign with ground- and space-based facilities from radio all the way up to the TeV range.
In this presentation I will give an overview of the results from this campaign. In 2017 we caught M 87 in a historically low state, both for its core and the nearest knot HST-1. In the high-energy range, at X-ray energies, the emission from the core dominates over HST-1. We present the simultaneously measured broad-band spectral energy distribution (SED) and discuss the complexity and caveats of combining data from different spatial scales. Two types of single-zone leptonic models assuming isotropic electron distribution functions and magnetic fields were used to investigate the basics emission properties. We conclude that these simple heuristic models are not able to simultaneously explain all of M 87’s observational properties, in contrast to many sources where a "single zone" can explain much of the SED. In particular, we can rule out that the observed low-state gamma-ray emission originates from the same region as the EHT mm-band emission. Furthermore, we find that the gamma-rays can only be produced in the inner jets (upstream of HST-1) if there are strongly particle-dominated regions, which may be the case for a jet sheath.
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