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

TL;DR

This study conducted extensive gamma-ray observations of the black hole binary MAXI J1820+070 during its 2018 outburst, setting upper limits on gamma-ray emission and constraining the properties of potential high-energy emission regions.

Contribution

First comprehensive gamma-ray observational constraints on MAXI J1820+070 during its outburst, combining data from multiple telescopes and multiwavelength observations to inform models of high-energy processes.

Findings

No gamma-ray emission detected; upper limits established.

Constraints on the location of high-energy emission regions.

Predicted gamma-ray fluxes are below current detection thresholds.

Abstract

MAXI J1820+070 is a low-mass X-ray binary with a black hole as a compact object. This binary underwent an exceptionally bright X-ray outburst from March to October 2018, showing evidence of a non-thermal particle population through its radio emission during this whole period. The combined results of 59.5 hours of observations of the MAXI J1820+070 outburst with the H.E.S.S., MAGIC and VERITAS experiments at energies above 200 GeV are presented, together with Fermi-LAT data between 0.1 and 500 GeV, and multiwavelength observations from radio to X-rays. Gamma-ray emission is not detected from MAXI J1820+070, but the obtained upper limits and the multiwavelength data allow us to put meaningful constraints on the source properties under reasonable assumptions regarding the non-thermal particle population and the jet synchrotron spectrum. In particular, it is possible to show that, if a high-energy gamma-ray emitting region is present during the hard state of the source, its predicted flux should be at most a factor of 20 below the obtained Fermi-LAT upper limits, and closer to them for magnetic fields significantly below equipartition. During the state transitions, under the plausible assumption that electrons are accelerated up to ~ 500 GeV, the multiwavelength data and the gamma-ray upper limits lead consistently to the conclusion that a potential high-energy and very-high-energy gamma-ray emitting region should be located at a distance from the black hole ranging between 10^11 and 10^13 cm. Similar outbursts from low-mass X-ray binaries might be detectable in the near future with upcoming instruments such as CTA.