Tetarenko, A. J.A. J.TetarenkoCASELLA, PiergiorgioPiergiorgioCASELLAMiller-Jones, J. C. A.J. C. A.Miller-JonesSivakoff, G. R.G. R.SivakoffPaice, J. A.J. A.PaiceVincentelli, F. M.F. M.VincentelliMaccarone, T. J.T. J.MaccaroneGandhi, P.P.GandhiDhillon, V. S.V. S.DhillonMarsh, T. R.T. R.MarshRussell, T. D.T. D.RussellUttley, P.P.Uttley2025-03-072025-03-0720210035-8711http://hdl.handle.net/20.500.12386/36470We present multiwavelength fast timing observations of the black hole X-ray binary MAXI J1820+070 (ASASSN-18ey), taken with the Karl G. Jansky Very Large Array (VLA), Atacama Large Millimeter/Sub-Millimeter Array (ALMA), Very Large Telescope (VLT), New Technology Telescope (NTT), Neutron Star Interior Composition Explorer (NICER), and XMM-Newton. Our data set simultaneously samples 10 different electromagnetic bands (radio - X-ray) over a 7-h period during the hard state of the 2018-2019 outburst. The emission we observe is highly variable, displaying multiple rapid flaring episodes. To characterize the variability properties in our data, we implemented a combination of cross-correlation and Fourier analyses. We find that the emission is highly correlated between different bands, measuring time-lags ranging from hundreds of milliseconds between the X-ray/optical bands to minutes between the radio/sub-mm bands. Our Fourier analysis also revealed, for the first time in a black hole X-ray binary, an evolving power spectral shape with electromagnetic frequency. Through modelling these variability properties, we find that MAXI J1820+070 launches a highly relativistic ($\Gamma =6.81^{+1.06}_{-1.15}$) and confined ($\phi =0.45^{+0.13}_{-0.11}$ deg) jet, which is carrying a significant amount of power away from the system (equivalent to $\sim 0.6 \, L_{1-100{\rm keV}}$). We additionally place constraints on the jet composition and magnetic field strength in the innermost jet base region. Overall, this work demonstrates that time-domain analysis is a powerful diagnostic tool for probing jet physics, where we can accurately measure jet properties with time-domain measurements alone.STAMPAenArticle10.1093/mnras/stab8202-s2.0-85114136130https://academic.oup.com/mnras/article/504/3/3862/61798772021MNRAS.504.3862TFIS/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