Accretion flow and jet in the hard state of MAXI J1820+070

A number of results on the accretion flow and the jet in the hard state of MAXI J1820+070 have been obtained. The accretion disc is found to have the inner radius of some tens of gravitational radii, Rg. The hot accretion flow, emitting hard X-rays, overlaps with the disc, and is stratified, consisting of at least two components with different spectral slopes. The harder component dominates the bolometric luminosity and is located downstream of the softer one. The electron distribution of the harder component is hybrid, consisting of mildly relativistic Maxwellian electrons followed by a power law tail. The spectra measured by NuSTAR and INTEGRAL extend up to ~2 MeV. This results in copious e± pair production in photon-photon collisions. The equilibrium pair density in the hot flow still remains low due to its relatively large optical depth. However, pairs are likely to be produced in the jet base, providing the electrons for the jet synchrotron emission. The jet parameters are determined based on the spectral and timing results of the large multiwalength campaign described in Tetarenko et al. 2021 (MNRAS, 504, 3862). The initial part of the jet forms a magnetically-dominated acceleration and collimation zone. The jet starts emitting above z0~104Rg. Its synchrotron emission by power-law electrons is partially self-absorbed up to the break frequency of ~(2−3)‰1013Hz. The half-opening angle is Θ ≈ 1−1.5°, the bulk Lorentz factor is G ≈ 1.8−4, and the magnetic field strength at z0 is B ≈ 104G. The jet power depends on the uncertain pair abundance, but it can be as large as its maximum value of Pj ≈ Ṁc2, where Ṁ is the accretion rate.