GRB 210528B

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.

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
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 ,
Lipunov et al. GCN Circular #30095
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 ) 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
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 )
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).

Alan M. Watson (UNAM), Nat Butler (ASU), Rosa L. Becerra (UNAM), Simone Dichiara
(GSFC/UMD), Alexander Kutyrev (GSFC/UMD), William H. Lee (UNAM), Oc��lotl 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 Astron��mico 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.