Multiwavelength Observations of the Black Hole X-ray Binary MAXI J1820$+$070 in the Rebrightening Phase

We report the results of quasi-simultaneous multiwavelength (near-infrared, optical, UV, and X-ray) observations of the Galactic X-ray black hole binary MAXI J1820+070 performed in 2019 May 10-13, $\sim 60$ days after the onset of the first rebrightening phase.

It showed a much larger optical-to-X-ray luminosity ratio ($\sim 8$) than in the initial outburst epoch.

The primary components of the spectral energy distribution (SED) can be best interpreted by radiatively inefficient accretion flow (RIAF) spectrum showing a luminosity peak in the optical band.

By comparison with theoretical calculations, we estimate the mass accretion rate to be $\dot{M}/(8 L_{\rm Edd}/c^2) \sim 10^{-3}$, where $c$ is the light speed and $L_{\rm Edd}$ is the Eddington luminosity.

In addition to the RIAF emission, a blue power-law component is detected in the optical-UV SED, which is most likely synchrotron radiation from the jet.

The optical spectrum taken at the Seimei telescope shows a weak and narrow H$\alpha$ emission line, whose emitting region is constrained to be $\gtrsim 2 \times 10^{4}$ times the gravitational radius.

We suggest that the entire disk structure cannot be described by a single RIAF solution but cooler material responsible for the H$\alpha$ emission must exist at the outermost region.

;Comment: 11 pages, 5 figures, Published in PASJ in 2022