[en] We present optical follow-up observation results of three binary black hole merger (BBH) events, GW190408_181802, GW190412, and GW190503_185404, which were detected by the Advanced LIGO and Virgo gravitational wave (GW) detectors. Electromagnetic (EM) counterparts are generally not expected for BBH merger events. However, some theoretical models suggest that EM counterparts of BBH can possibly arise in special environments, prompting motivation to search for EM counterparts for such events. We observed high-credibility regions of the sky for the three BBH merger events with telescopes of the Gravitational-wave EM Counterpart Korean Observatory (GECKO), including the KMTNet. Our observation started as soon as 100 minutes after the GW event alerts and covered 29–63 deg² for each event with a depth of ∼22.5 mag in the R band within hours of observation. No plausible EM counterparts were found for these events, but based on there being no detection of the GW190503_185404 event, for which we covered the 69% credibility region, we place the BBH merger EM counterpart signal to be M _g > − 18.0 AB mag within about one day of the GW event. The comparison of our detection limits with light curves of several kilonova models suggests that a kilonova event could have been identified within hours of the GW alert with GECKO observations if the compact merger happened at <400 Mpc and the localization accuracy was on the order of 50 deg². Our result shows great promise for the GECKO facilities to find EM counterparts within a few hours from GW detection in future GW observation runs.

[en] The asteroid exploration project “Hayabusa2” has successfully returned samples from the asteroid (162173) Ryugu. In this study, we measured the linear polarization degrees of Ryugu using four ground-based telescopes from 2020 September 27 to December 25, covering a wide-phase angle (Sun-target-observer’s angle) range from 28° to 104°. We found that the polarization degree of Ryugu reached 53% around a phase angle of 100°, the highest value among all asteroids and comets thus far reported. The high polarization degree of Ryugu can be attributed to the scattering properties of its surface layers, in particular the relatively small contribution of multiply scattered light. Our polarimetric results indicate that Ryugu’s surface is covered with large grains. On the basis of a comparison with polarimetric measurements of pulverized meteorites, we can infer the presence of submillimeter-sized grains on the surface layer of Ryugu. We also conjecture that this size boundary represents the grains that compose the aggregate. It is likely that a very brittle structure has been lost in the recovered samples, although they may hold a record of its evolution. Our data will be invaluable for future experiments aimed at reproducing the surface structure of Ryugu.