Evolution of accretion disk structure of the black hole X-ray binary MAXI J1820+070 during the rebrightening phase

To understand the evolution of global accretion disk structure in the "rebrightening" phase of MAXI J1820+070, we perform a comprehensive analysis of its near infrared/optical/UV to X-ray spectral energy distribution (SED) utilizing data obtained by OISTER, Las Cumbres Observatory (LCO), Swift, NICER, and NuSTAR in 2019. Optical spectra observed with Seimei telescope in 2019 and 2020 are also analyzed. On the basis of the optical and X-ray light curves and their flux ratios, we divide the whole phase into three periods, Periods I (flux rise), II (decay), and III (dim). In the first two periods, the source stayed in the low/hard state (LHS), where the X-ray (0.3-30 keV) and optical/UV SED can both be fitted with power-law models. We interpret that the X-ray emission arises from hot corona via Comptonization, whereas the optical/UV flux is dominated by synchrotron radiation from the jets, with a partial contribution from the irradiated disk. The optical/UV power-law component smoothly connects to a simultaneous radio flux, supporting its jet origin. Balmer line profiles in the optical spectra indicate that the inner radius of an irradiated disk slightly decreased from ~2 × 10⁵r_g (Period I) to ~1 × 10⁵r_g (Period II), where r_g is the gravitational radius, implying a change of the hot corona geometry. In Period III, the SED can be reproduced by an advection-dominated accretion flow and jet emission. However, the double-peaked Hα emission line indicates that a cool disk remained at large radii.