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MAXI J1820+070 is a low-mass X-ray binary with a black hole as a compactobject. This binary underwent an exceptionally bright X-ray outburst from Marchto October 2018, showing evidence of a non-thermal particle population throughits radio emission during this whole period. The combined results of 59.5 hoursof observations of the MAXI J1820+070 outburst with the H.E.S.S., MAGIC andVERITAS experiments at energies above 200 GeV are presented, together withFermi-LAT data between 0.1 and 500 GeV, and multiwavelength observations fromradio to X-rays. Gamma-ray emission is not detected from MAXI J1820+070, butthe obtained upper limits and the multiwavelength data allow us to putmeaningful constraints on the source properties under reasonable assumptionsregarding the non-thermal particle population and the jet synchrotron spectrum.In particular, it is possible to show that, if a high-energy gamma-ray emittingregion is present during the hard state of the source, its predicted fluxshould be at most a factor of 20 below the obtained Fermi-LAT upper limits, andcloser to them for magnetic fields significantly below equipartition. Duringthe state transitions, under the plausible assumption that electrons areaccelerated up to ~ 500 GeV, the multiwavelength data and the gamma-ray upperlimits lead consistently to the conclusion that a potential high-energy andvery-high-energy gamma-ray emitting region should be located at a distance fromthe black hole ranging between 10^11 and 10^13 cm. Similar outbursts fromlow-mass X-ray binaries might be detectable in the near future with upcominginstruments such as CTA.

Gamma-ray observations of MAXI J1820+070 during the 2018 outburst