A Multi-wavelength View of M87 during the First EHT Campaign

In the 2017 observing campaign, the Event Horizon Telescope (EHT) Collaboration partnered with several international facilities in space and on the ground, to arrange extensive, quasi-simultaneous, multi-wavelength coverage of its science targets. The EHT observations resulted in the first direct image of the center of the M87 galaxy, featuring asymmetric ring morphology and size consistent with theoretical expectations for a weakly accreting supermassive black hole. The quasi-simultaneous multi-wavelength data publicly released in 2021 and presented in this talk further support the physical interpretation and extend the modeling down the relativistic jet. We captured M87 in a historically low state, when the core flux dominated over the HST-1 knot at high energies, making it possible to combine core flux constraints with the more spatially precise very long baseline interferometry data. There are notable complexities and caveats of combining data from different spatial scales into a single spectrum. To aid the interpretation and provide insight into the basic source properties, we applied two different heuristic, isotropic, leptonic, single-zone models, leading to the conclusion that the broadband spectrum of M87 points to a structured jet. We can exclude that the simultaneous gamma-ray emission is produced via inverse Compton emission in the same region producing the millimeter-band emission observed with the EHT. While direct synchrotron emission from accelerated protons and secondaries cannot yet be excluded, the gamma-rays can only be produced in the inner jets (inward of HST-1) if there are strongly particle-dominated regions. Further insight is expected from the comparison of multi-wavelength behavior between the 2017 and 2018 observing campaigns, which should be especially interesting due to different activity states of M87 at high energies.