Multiwavelength observations of the black hole X-ray binary MAXI J1820+070 in the rebrightening phase

• Published in: Publications of the Astronomical Society of Japan Vol. 74; no. 4; pp. 805 - 814
• Main Authors: Yoshitake, Tomohiro, Shidatsu, Megumi, Ueda, Yoshihiro, Mineshige, Shin, Murata, Katsuhiro L, Adachi, Ryo, Maehara, Hiroyuki, Nogami, Daisaku, Negoro, Hitoshi, Kawai, Nobuyuki, Niwano, Masafumi, Hosokawa, Ryohei, Saito, Tomoki, Oasa, Yumiko, Takarada, Takuya, Shigeyoshi, Takumi
• Format: Journal Article
• Language: English
• Published: Oxford University Press 04.08.2022
• Subjects: X-rays: individual (MAXI J1820+070); black hole physics; X-rays: binaries; accretion, accretion disks
• ISSN: 0004-6264; 2053-051X
• DOI: 10.1093/pasj/psac038

Abstract 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, ∼60 d after the onset of the first rebrightening phase. It showed a much larger optical-to-X-ray luminosity ratio (∼8) than in the initial outburst epoch. The primary components of the spectral energy distribution (SED) can be best interpreted by a 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 LEdd 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 Hα emission line, the emitting region of which is constrained to be ≳2 × 104 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 Hα emission must exist at the outermost region.