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.