Jet Parameters in the Black Hole X-Ray Binary MAXI J1820+070

We study the jet in the hard state of the accreting black hole (BH) binary MAXI J1820+070. From the available radio-to-optical spectral and variability data, we put strong constraints on the jet parameters. We find while it is not possible to uniquely determine the jet Lorentz factor from the spectral and variability properties alone, we can estimate the jet opening angle (approximate to 1.degrees 5 +/- 1 degrees), the distance at which the jet starts emitting synchrotron radiation (similar to 3 x 10(10) cm), and the magnetic field strength there (similar to 10(4) G), with relatively low uncertainty, as they depend weakly on the bulk Lorentz factor. We find the breaks in the variability power spectra from radio to submillimeter wavelength are consistent with variability damping over the timescale equal to the travel time along the jet at any Lorentz factor. This factor can still be constrained by the electron-positron pair-production rate within the jet base, which we calculate based on the observed X-ray/soft-gamma-ray spectrum, and the jet power, required to be less than the accretion power. The minimum (similar to 1.5) and maximum (similar to 4.5) Lorentz factors correspond to the dominance of pairs and ions, and the minimum and maximum jet power, respectively. We estimate the magnetic flux threading the BH and find the jet can be powered by the Blandford-Znajek mechanism in a magnetically arrested flow accretion flow. We point out the similarity of our derived formalism to that of core shifts, observed in extragalactic radio sources.

Automated summary

By studying the jet of the accreting black hole binary MAXI J1820+070, significant constraints on jet parameters such as opening angle, emission distance, and magnetic field strength have been obtained. Variability power spectrum breaks are consistent with travel time along the jet at different Lorentz factors. The jet's power source is identified as magnetically arrested flow accretion, similar to core shifts observed in extragalactic radio sources.