A Broadband View on Microquasar MAXI J1820+070 during the 2018 Outburst

The microquasar MAXI J1820+070 went into outburst from 2018 mid-March until mid-July, with several faint rebrightenings afterward. 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 multiwavelength observations, we analyzed quasi-simultaneous observations from April 12, near the peak of the outburst (∼March 23). Analysis of the X-ray spectrum found it indicative of an accreting black hole binary in the hard state, consistent with the flat/inverted radio spectrum and the accretion disk winds seen at optical wavelengths. Then, we constructed a spectral energy distribution spanning ∼12 orders of magnitude using modeling in 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. JetSeT finds that the spectrum is dominated by jet emission up to approximately 10¹⁴ Hz, after which disk and coronal emission dominates. The acceleration region has a magnetic field of B ∼ 1.6 × 10⁴ G, a cross section of R ∼ 2.8 × 10⁹ cm, and a flat radio spectral shape naturally obtained from the synchroton cooling of the accelerated electrons. The jet luminosity is >8 × 10³⁷ erg s^-1 (>0.15L_Edd), compared to an accretion luminosity of ∼6 × 10³⁷ erg s^-1, assuming a distance of 3 kpc. Because these two values are comparable, it is possible that 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.