//////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 30067 SUBJECT: GRB 210521A: MAXI/GSC detection DATE: 21/05/21 15:04:39 GMT FROM: Motoko Serino at Aoyama Gakuin U. S. Yamada (Kyoto U.), 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, Y. Sugawara, S. Ueno, H. Tomida, M. Ishikawa, M. Tominaga, T. Nagatsuka (JAXA), Y. Ueda, 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 13:04:08 UT on 2021 May 21. Assuming that the source flux was constant over the transit, we obtain the source position at (R.A., Dec) = (264.674 deg, -61.752 deg) = (17 38 41, -61 45 07) (J2000) with a 90% C.L. statistical error of 0.12 deg and an additional systematic uncertainty of 0.1 deg (90% containment radius). The X-ray flux averaged over the scan was 508 +- 51 mCrab (4.0-10.0keV, 1 sigma error). Without assumptions on the source constancy, we obtain a rectangular error box for the transient source with the following corners: (R.A., Dec) = (264.912, -61.246) deg = (17 39 38, -61 14 45) (J2000) (R.A., Dec) = (264.426, -61.253) deg = (17 37 42, -61 15 10) (J2000) (R.A., Dec) = (264.502, -62.494) deg = (17 38 00, -62 29 38) (J2000) (R.A., Dec) = (265.008, -62.486) deg = (17 40 01, -62 29 09) (J2000) There was no significant excess flux in the previous transit at 12:43 UT and in the next transit at 14:16 UT with an upper limit of 20 mCrab for each. //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 30068 SUBJECT: MAXI GRB210521.54: Global MASTER-Net observations report DATE: 21/05/22 04:51:15 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs V. Lipunov, E. Gorbovskoy, V.Kornilov, N.Tyurina, P.Balanutsa, A.Kuznetsov, F.Balakin, V.Vladimirov, D. Vlasenko, I.Gorbunov, D.Zimnukhov, V.Senik, T.Pogrosheva, D.Kuvshinov, D. Cheryasov (Lomonosov Moscow State University, SAI, Physics Department), R. Podesta, C.Lopez, F. Podesta, C.Francile (Observatorio Astronomico Felix Aguilar OAFA), R. Rebolo, M. Serra (The Instituto de Astrofisica de Canarias), D. Buckley (South African Astronomical Observatory), O.A. Gres, N.M. Budnev, O.Ershova (Irkutsk State University, API), A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory), V. Yurkov, A. Gabovich, Yu. Sergienko (Blagoveschensk Educational State University) MASTER-SAAO robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in South Africa (South African Astronomical Observatory) started inspect of the MAXI GRB210521.54 (trigger No 355431084,17h 38m 53.52s , -61d 22m 44.4s, R=1) errorbox 51456 sec after notice time and 56029 sec after trigger time at 2021-05-22 04:38:07 UT, with upper limit up to 15.6 mag. Observations started at twilight. The observations began at zenith distance = 50 deg. The sun altitude is -10.1 deg. The galactic latitude b = -16 deg., longitude l = 331 deg. Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=1618748 We obtain a following upper limits. Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment _________|_____________________|_____________________|____________________________________|_____|_______|_______|________ 56060 | 2021-05-22 04:38:07 | MASTER-SAAO | (17h 38m 08.44s , -61d 25m 58.7s) | C | 60 | 15.6 | Filter C is a clear (unfiltred) band. The observation and reduction will continue. The message may be cited.