Homan, JeroenJeroenHomanBright, JoeJoeBrightMOTTA, Sara ElisaSara ElisaMOTTAAltamirano, DiegoDiegoAltamiranoArzoumanian, ZavenZavenArzoumanianBasak, ArkadipArkadipBasakBELLONI, Tomaso Maria MelchiorreTomaso Maria MelchiorreBELLONICackett, Edward M.Edward M.CackettFender, RobRobFenderGendreau, Keith C.Keith C.GendreauKara, ErinErinKaraPasham, Dheeraj R.Dheeraj R.PashamRemillard, Ronald A.Ronald A.RemillardSteiner, James F.James F.SteinerStevens, Abigail L.Abigail L.StevensUttley, PhilPhilUttley2025-03-072025-03-0720202041-8205http://hdl.handle.net/20.500.12386/36545We present Neutron Star Interior Composition Explorer X-ray and Arcminute Microkelvin Imager Large Array radio observations of a rapid hard-to-soft state transition in the black hole X-ray transient MAXI J1820+070. During the transition from the hard state to the soft state a switch between two particular types of quasiperiodic oscillations (QPOs) was seen in the X-ray power density spectra, from type-C to type-B, along with a drop in the strength of the broadband X-ray variability and a brief flare in the 7-12 keV band. Soon after this switch (∼2-2.5 hr) a strong radio flare was observed that corresponded to the launch of superluminal ejecta. Although hints of a connection between QPO transitions and radio flares have been seen in other black hole X-ray transients, our observations constitute the strongest observational evidence to date for a link between the appearance of type-B QPOs and the launch of discrete jet ejections.STAMPAenArticle10.3847/2041-8213/ab79322-s2.0-85085149938https://api.elsevier.com/content/abstract/scopus_id/85085149938https://iopscience.iop.org/article/10.3847/2041-8213/ab7932FIS/05 - ASTRONOMIA E ASTROFISICAERC sectors::Physical Sciences and Engineering::PE9 Universe sciences: astro-physics/chemistry/biology; solar systems; stellar, galactic and extragalactic astronomy, planetary systems, cosmology, space science, instrumentation