The Accretion-Ejection Connection in the Black Hole X-ray Binary MAXI J1820$+$070

Abstract

The black hole X-ray binary MAXI J1820$+$070 began its first recorded outburst in March 2018, and remained an active radio, X-ray, and optical source for over four years. Due to the low distance to the source and its intrinsically high luminosity MAXI J1820$+$070 was observed extensively over this time period, resulting in high-cadence and quasi-simultaneous observations across the electromagnetic spectrum. These data sets provide the opportunity to probe the connection between accretion and the launch of jets in greater detail than for the majority of black hole X-ray binaries. In this work we present radio (Arcminute Microkelvin Imager Large Array, MeerKAT), X-ray (Swift), and optical (Las Cumbres Observatory) observations of MAXI J1820$+$070 throughout its entire outburst, including its initial hard state, subsequent soft state, and further hard-state-only re-brightenings (covering March 2018 to August 2022). Due to the regularity and temporal density of our observational data we are able to create a Radio - X-ray - Optical activity plane where we find a high degree of correlation between the three wave bands during the hard states, and observe hysteresis as MAXI J1820$+$070 enters and exits the soft state. Based on the morphology of the optical light curves we see evidence for optical jet contributions during the soft-to-hard state transition, as well as fading optical emission well before the hard to soft transition. We establish that the remarkably similar profiles of the re-brightening events are broadly consistent with modified disk instability models where irradiation from the inner accretion disk is included.