TITLE: GCN CIRCULAR NUMBER: 32949 SUBJECT: GRB 221009A: Japanese VLBI Network observation DATE: 22/11/19 04:37:23 GMT FROM: Kotaro Niinuma at Yamaguchi University ========================================================================= K. Niinuma (Yamaguchi Univ.), Y. Yonekura (Ibaraki Univ.), K. Fujisawa, K. Motogi (Yamaguchi Univ.), and W. Iwakiri (Chiba Univ.) We carried out the very long baseline interferometry (VLBI) observations of GRB221009A/Swift J1913.1+1946 (Dichiara et al. GCN 32632, ATel 15650) at 9:00 - 10:15 UT on October 11, at 8:00 - 9:15 on October 25 and 26, 2022 (1.82 days, 15.78 days, and 16.78 days after the Fermi-GBM trigger (Veres et al. GCN 32632), respectively). The observation was performed in both C-band (center frequency of 6856 MHz with a bandwidth of 512 MHz) and X-band (center frequency of 8448 MHz with a bandwidth of 512 MHz), simultaneously by single baseline interferometry consisting of Yamaguchi-34m radio telescope operated by Yamaguchi University and Hitachi-32m radio telescope operated by Ibaraki University. Both telescopes recorded only left-circular polarization. This array is a part of the Japanese VLBI Network and the baseline length is of 873 km. GRB221009A/Swift J1913.1+1946 was successfully detected on October 11, and its VLBI flux densities were 13+/–3 mJy at 6.86 GHz and of 11+/-3 mJy at 8.45 GHz. On the other hand, it was not detected on October 25 and 26, and the 5-sigma upper limits of VLBI flux density were 3mJy at both 6.86GHz and 8.45GHz. The bright quasar J1905+1943 was also observed to determine the VLBI flux density of GRB221009A/Swift J1913.1+1946. The radio emission from the very compact component detected by our VLBI observation showed slightly steep spectrum in radio band on 1.82 days after the Fermi-GBM trigger. ========================================================================= -- NIINUMA Kotaro Graduate School of Sciences and Technology for Innovation, Yamaguchi Univ., Yoshida 1677-1, Yamaguchi 753-8512, Japan Phone: +81-(0)83-933-5759 E-mail:niinuma@yamaguchi-u.ac.jp 新沼 浩太郎 山口大学大学院 創成科学研究科 〒753-8512 山口県山口市吉田1677−1