//////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 30094 SUBJECT: GRB 210528B: MAXI/GSC detection DATE: 21/05/29 00:38:27 GMT FROM: Motoko Serino at Aoyama Gakuin U. Y. Sugawara (JAXA), H. Negoro, M. Nakajima, K. Kobayashi, K. Asakura, K. Seino (Nihon U.), T. Mihara, T. Tamagawa, M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, S. Sugita, K. Komachi, A. Yoshida (AGU), Y. Tsuboi, W. Iwakiri, H. Kawai, Y. Okamoto, S. Kitakoga (Chuo U.), M. Shidatsu (Ehime U.), N. Kawai, M. Niwano, R. Hosokawa (Tokyo Tech), S. Nakahira, S. Ueno, H. Tomida, M. Ishikawa, M. Tominaga, T. Nagatsuka (JAXA), Y. Ueda, S. Yamada, S. Ogawa, K. Setoguchi, T. Yoshitake, Y. Goto, R. Uematsu (Kyoto U.), H. Tsunemi (Osaka U.), M. Yamauchi, Y. Nonaka, T. Sato, R. Hatsuda, R. Fukuoka (Miyazaki U.), T. Kawamuro (UDP/NAOJ), K. Yamaoka (Nagoya U.), Y. Kawakubo (LSU), M. Sugizaki (NAOC) report on behalf of the MAXI team: The MAXI/GSC nova alert system triggered a bright uncatalogued X-ray transient source at 22:51:37 UT on 2021 May 28. Assuming that the source flux was constant over the transit, we obtain the source position at (R.A., Dec) = (239.539 deg, 5.983 deg) = (15 58 09, +05 58 58) (J2000) with a statistical 90% C.L. elliptical error region with long and short radii of 0.26 deg and 0.22 deg, respectively. The roll angle of long axis from the north direction is 133.0 deg counterclockwise. There is an additional systematic uncertainty of 0.1 deg (90% containment radius). Without assumptions on the source constancy, we obtain a rectangular error box for the transient source with the following corners: (238.974, 6.826) deg = (15 55 53, +06 49 33) (J2000) (238.661, 6.518) deg = (15 54 38, +06 31 04) (J2000) (240.060, 5.113) deg = (16 00 14, +05 06 46) (J2000) (240.373, 5.421) deg = (16 01 29, +05 25 15) (J2000) The X-ray flux averaged over the scan was 170 +- 28 mCrab (4.0-10.0keV, 1 sigma error). There was no significant excess flux in the previous transit at 21:18 UT and in the next transit at 00:24 on May 29 UT with an upper limit of 20 mCrab for each. //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 30096 SUBJECT: GRB 210528B: MITSuME Akeno optical upper limits DATE: 21/05/29 13:57:33 GMT FROM: Katsuhiro L. Murata at Nagoya U K. L. Murata, R. Hosokawa, M. Niwano, Y. Imai, N. Ito, H. Takamatsu, S. Sato, M. Takaku, R. Noto, R. Yamaguchi, Y. Yatsu, and N. Kawai (TokyoTech) report on behalf of the MITSuME collaboration: We observed the field of GRB 210528B (Sugawara et al. GCN Circular #30094, Lipunov et al. GCN Circular #30095) with the optical three color (g', Rc, and Ic) CCD cameras attached to the MITSuME telescope Akeno. The observation with a series of 60 sec exposures started at 2021-05-29 11:21:57 UT (12.5 hours after the trigger). We stacked the images with good conditions. We did not detect any uncatalogued sources in all three bands within the elliptical error region (Sugawara et al. GCN Circular #30094) determined under the assumption that the source flux was constant over the transit. We obtained the 5-sigma limits of the stacked images as follows. T0+[hour] MID-UT T-EXP[sec] 5-sigma limits ------------------------------------------------------------------------------------------------ 12.5 2021-05-29 12:13 1560 g'>19.9 12.5 2021-05-29 12:13 2040 Rc>20.2 12.5 2021-05-29 12:13 1500 Ic>19.4 ------------------------------------------------------------------------------------------------ T0+ : Elapsed time after the burst T-EXP: Total Exposure time We used the PS1 catalog for flux calibration. The magnitudes are expressed in the AB system. The images were processed in real-time through the MITSuME GPU reduction pipeline (Niwano et al. 2021, PASJ, Vol.73, Issue 1, Pages 4-24; https://github.com/MNiwano/Eclaire). //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 30097 SUBJECT: GRB 210528B: DDOTI Upper Limits on the Afterglow DATE: 21/05/29 14:32:38 GMT FROM: Alan M Watson at UNAM Alan M. Watson (UNAM), Nat Butler (ASU), Rosa L. Becerra (UNAM), Simone Dichiara (GSFC/UMD), Alexander Kutyrev (GSFC/UMD), William H. Lee (UNAM), Océlotl Lopez (UNAM), Diego Gonzalez (UNAM), Margarita Pereyra (UNAM), Srihari Ravi (ASU), and Eleonora Troja (GSFC/UMD) report: We observed the field of the MAXI/GSC GRB 210528B (Sugawara et al., GCN Circ., 30094) with the DDOTI wide-field imager at the Observatorio Astronómico Nacional on Sierra San Pedro Martir (http://ddoti.astroscu.unam.mx) from 2021-05-29 04:07 to 06:53 UTC (from 5.3 to 8.0 hours after the trigger). We observed a region covering aproximately 7 degrees in RA and 10 degrees in declination (1 instrumental field or about 70 square degrees) including the entire MAXI/GSC uncertainty region centered on 15:58:09 +05:58:58 J2000 (Sugawara et al., GCN Circ., 30094). We obtained 7860 seconds of exposure in the w filter. We obtained AB photometry by calibration against the APASS catalog. Within the uncertainty region, we detect no likely candidate for the afterglow to our 5-sigma upper limits of w = 21.8. We thank the staff of the Observatorio Astronomico Nacional in San Pedro Martir.