Do you only use radio telescopes or are you also looking at higher-energy emission? What makes radio telescopes superior to, for example, X-ray telescopes?

Scientists of the EHT and their collaborators try to organize observations with a number of different telescopes so that they coincide with observations with EHT observations. The aim of this is to provide multi-wavelength coverage in order to investigate potential correlations in source brightness in various bands across the electromagnetic spectrum. In 2017 and 2018, coordinated observations were performed by various radio telescope arrays operating at wavelengths longer than 1 mm, such as GMVA, VLBA, KVN, HSA, EVN, RadioAstron. A number of optical and infrared telescopes monitored the EHT targets, as did X-ray telescopes Chandra, Swift, NuSTAR, and AstroSat, and high-energy gamma-ray observatories MAGIC, VERITAS, and HESS.

What makes the radio observations at 1 mm wavelength different from observations in any other wavelength band is the spatial resolution. Only observations with the Earth-sized EHT array at wavelength below approximately 2 mm have the theoretical resolving power sufficient to discern the very small size of the event horizons of black holes in SgrA* and M87. Resolving power scales directly with the observing wavelength and inversely with the distance between the furthest telescopes in an interferometric array such as the EHT. Other radio arrays, including global ones like the GMVA, do not operate at short enough wavelengths, while at wavelengths shorter than about a tenth of a millimeter long-distance interferometry becomes technically impossible. As a result, telescopes at other wavelengths either view changes on spatial scales larger than those observed with the EHT, or have a difficulty in discerning where exactly (with respect to the black hole) the changes are happening.