Multiwavelength Observations of the Black Hole X-ray
Binary MAXI J1820+070 in the Rebrightening Phase
T. Yoshitake1, M. Shidatsu2, Y. Ueda1, S. Mineshige1, K. L Murata1,3, R. Adachi3, H. Maehara4, D. Nogami1, H. Negoro5,3336777
N. Kawai , M. Niwano , R. Hosokawa , T. Saito , Y. Oasa , T. Takarada , T. Shigeyoshi , OISTER Collaboration
1: Kyoto University 2: Ehime University 3: Tokyo Tech 4: Okayama Observatory/NAOJ 5: Nihon University
6: Nishi-Harima Astronomical Observatory 7:Saitama UniversityWe report the results of quasi-simultaneous multiwavelength (near-infrared, optical, UV, and X-ray) observations of the Galactic X-ray blackhole binary MAXI J1820+070 performed in 2019 May 10–13, ∼60 days 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 bebest interpreted by a radiatively inefficient accretion flow (RIAF) spectrum showing a luminosity peak in the optical band. By comparison with2)∼10−3, where c is the light speed and L̇theoretical calculations, we estimate the mass accretion rate to be 𝑀𝑀/(8L/c
Edd
Edd is the Eddingtonluminosity. 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 ofwhich 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 must exist at the outermost region. Published as Yoshitake et al. (2022), PASJ, 74, 4

We observed MAXI J1820+070 (Kawamuro et al. 2018) with Swift/XRT (X-ray),
Swift/UVOT (UV), and OISTER (near-infrared to optical) on 2019 May 10–13.
Optical spectroscopic observations were performed with KOOLS-IFU on Seimei
telescope on 2019 May 11.
Figure 1 shows the long-term X-ray (MAXI) and optical (OISTER) light curve
over three years since its discovery. Our multi-wavelength observation wasperformed in the last part of the decay phase of the first rebrightening, with the g′band magnitude of ∼16 mag. The optical-to-X-ray luminosity ratio (∼8) is found to
be much larger than those observed in an earlier outburst epoch when the source
was in the LHS or HSS (Shidatsu et al. 2018; 2019), suggesting a very different
disk structure.

Model 2 (Truncated Hot StandardDisk)
As an alternative model, a major partof the optical SED could bereproduced by optically thickemission from a standard disk
truncated at a very large radius (~104rg, where rg is the gravitational radius,Figure 3). We conclude that this“truncated hot standard disk”
scenario is unlikely, because theinferred mass accretion rate from themulti-color disk model, 𝑚𝑚̇ ~0.04, is
too large to explain the observed
optical and X-ray luminosities,
assuming that the inner accretion disk
becomes RIAF.
Fig. 3. Same as Figure 2, but for the redden*(diskbb+bbodyrad) model in the middle panel and for theredden*(powerlaw+diskbb+bbodyrad) model (Model 2) in the top and bottom panels, in the NIR to UV band. Thebbodyrad model (common to Fig. 2) represents the blackbody emission from the companion star (K4V star; Torres etal. 2020), whose temperature and radius are assumed to be 4700 K and 0.65 solar radius, respectively.

Fig. 1. (Top) g′-band optical light curve of MAXI J1820 from the OISTER collaboration (R. Adachi et al. in preparation).
(Bottom) X-ray light curve in 2–10 keV from the MAXI/GSC. MJD 58200 corresponds to 2019 March 23. The yellow
line presents our observation epoch (2019 May 11 = MJD 58614).

The optical spectrum observed with KOOLS-IFU on the Seimei telescope (Figure4) shows a weak Hα emission line with an FWHM of ∼5.3 ×102 km s−1. The inner
radius of the Hα-emitting region is constrained to be ≳ 2 × 104 rg. We suggest thatthe entire disk structure cannot be described by a single RIAF solution but coolermaterial must exist at the outermost region.

We considered two possibilities as the primary origin of the IR-to-UV
SED: (Model 1) RIAF and (Model 2) truncated hot standard disk. In addition to theprimary component, a blue power-law component dominating the UV flux was̈
detected (see also Ozbey
Arabaci et al. 2022). We interpret that this is produced bysynchrotron radiation by non-thermal electrons in the jet, as suggested by its rapid
variability (R. Adachi et al. in preparation).
Model 1 (RIAF)
The primary components of the opticaland X-ray SED can be best interpretedby an RIAF spectrum (Figure 2),
showing a luminosity peak in the opticalband (via thermal synchro-cyclotron)
and a power-law in the X-ray band (viathermal bremsstrahlung). By comparison
with theoretical calculations (Manmotoet al. 1997; Mahadevan 1997), weestimate the mass accretion rate to be 𝑚𝑚̇̇ 2/8LEdd) ~10−3⁠, which reproduces(≡ 𝑀𝑀cthe observed optical and X-ray
luminosities within factors of 6.
Fig. 2. Top: SED data with the best-fitting redden*(powerlaw+cutoffpl+bbodyrad) model (Model 1) in the NIR to UV band andthe unfolded spectrum with the best-fitting TBabs*power-law model in X-rays, all corrected for the interstellar absorption andextinction. The individual components are separately plotted. Middle: the data versus model ratio of theredden*(cutoffpl+bbodyrad) model (in the infrared to UV band) and the TBabs*power-law model (in the X-ray band). Bottom:
Same as the middle panel, but with the redden*(powerlaw+cutoffpl+bbodyrad) model in the infrared to UV band.

Fig. 4. Seimei/KOOLS-IFU spectrum of MAXI J1820, taken on 2019 May 11. The magnified image presents theSeimei spectrum around the Hα line and the best-fitting diskline model.

[1] Yoshitake et al. 2022, PASJ, 74, 4[2] Kawamuro et al. 2018, ATel 11399, 1[3] Shidatsu et al. 2018, ApJ, 868, 54[4] Shidatsu et al. 2019, ApJ, 874, 183
̈[5] 𝐎𝐎zbey
Arabaci et al. 2022, MNRAS, 514, 3894

[6] Manmoto et al. 1997, ApJ, 489, 791[7] Mahadevan 1997, ApJ, 477, 585[8] Torres et al. 2020, ApJ, 893, L37