  The microquasar MAXI J\(1820+070\) went into outburst from mid-March until
mid-July 2018 with several faint rebrightenings afterwards. With a peak flux of
approximately 4 Crab in the \(20-50\) keV, energy range the source was
monitored across the electromagnetic spectrum with detections from radio to
hard X-ray frequencies. Using these multi-wavelength observations, we analyzed
quasi-simultaneous observations from 12 April, near the peak of the outburst
(\(\sim 23\) March). Spectral analysis of the hard X-rays found a \(kT_e \sim
30 \) keV and \( \tau \sim 2\) with a \texttt{CompTT} model, indicative of an
accreting black hole binary in the hard state. The flat/inverted radio spectrum
and the accretion disk winds seen at optical wavelengths are also consistent
with the hard state. Then we constructed a spectral energy distribution
spanning \(\sim 12\) orders of magnitude using modelling in \texttt{JetSeT}.
The model is composed of an irradiated disk with a Compton hump and a leptonic
jet with an acceleration region and a synchrotron-dominated cooling region.
\texttt{JetSeT} finds the spectrum is dominated by jet emission up to
approximately \(10^{14}\) Hz after which disk and coronal emission dominate.
The acceleration region has a magnetic field of \( B \sim 1.6 \times 10^4 \) G,
a cross section of \(R \sim 2.8 \times 10^{9} \) cm, and a flat radio spectral
shape naturally obtained from the synchroton cooling of the accelerated
electrons. The jet luminosity of \(> 8 \times 10^{37} \) erg/s (\(>
0.15L_{Edd}\)) compared to an accretion luminosity of \( \sim 6 \times
10^{37}\) erg/s, assuming a distance of 3 kpc. Because these two values are
comparable, it is possible the jet is powered predominately via accretion with
only a small contribution needed from the Blanford-Znajek mechanism from the
reportedly slowly spinning black hole.
