Simultaneous Multicolor Photometry of the DESTINY\(^{+}\) target asteroid (3200) Phaethon

Accurate estimation of brightness of (3200) Phaethon up to lower phase angles are essential for planning of the on-board camera of the DESTINY\(^{+}\) mission. We have carried out intensive observations of Phaethon in the optical wavelength (\(g\), \(r\), and \(i\)) with the TriCCS camera on the Seimei 3.8 m telescope in October and November, 2021. We derived the absolute magnitude \(H_\mathrm{V}\) and the slope parameter \(G\) of Phaethon as \(H_\mathrm{V}=14.23\pm0.02\) and \(G=0.040\pm0.008\) from multiple photometric observations including lower phase angles down to \(\sim\)9\(^{\circ}\) with the \(H\)-\(G\) model. Using the \(H_\mathrm{V}\) value and the geometric albedo of Phaethon derived in previous polarimetric studies, we estimated that the Phaethon's diameter is within a range of 5.22 to 6.74 km, which is consistent with radar and occultation observations. With the linear model, we derived \(H_\mathrm{V}=14.65\pm0.02\), which corresponds to a diameter range of 4.30 to 5.56 km. Our simultaneous tricolor lightcurves of Phaethon indicate that no rotational spectral variations larger than 0.018 and 0.020 mag in the g-r and r-i colors, possibly related to inhomogeneity of the surface material and/or structure, are seen at the 2021 apparition.