A broadband view on microquasar MAXI J1820+070 during the 2018 outburst

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 $\mathtt{\text{CompTT}}$ 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 $\mathtt{\text{JetSeT}}$. 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. $\mathtt{\text{JetSeT}}$ 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.