GRB 211225A

T. Kurihara (JAXA), H. Negoro, M. Nakajima, K. Kobayashi, K. Asakura, K. Seino (Nihon U.), 
T. Mihara, T. Tamagawa, J. Li, M. Matsuoka (RIKEN), 
T. Sakamoto, M. Serino, S. Sugita, K. Komachi, H. Hiramatsu, A. Yoshida (AGU), 
Y. Tsuboi, W. Iwakiri,  H. Kawai, Y. Okamoto, S. Kitakoga (Chuo U.), 
M. Shidatsu, M. Iwasaki (Ehime U.), 
N. Kawai, M. Niwano, R. Hosokawa, Y. Imai, N. Ito, Y. Takamatsu (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, K. Inaba (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 on a bright uncatalogued X-ray transient source 
at 03:22:28 UT on 2021 December 25.
Assuming that the source flux was constant over the transit,
we obtain the source position at
(R.A., Dec) = (126.923 deg, -7.182 deg) = (08 27 41, -07 10 55) (J2000) 
with a statistical 90% C.L. elliptical error region 
with long and short radii of 0.20 deg and 0.17 deg, respectively. 
The roll angle of long axis from the north direction is 40.0 deg counterclockwise. 
Without assumptions on the source constancy, we obtain a rectangular error
box for the transient source with the following corners:
(R.A., Dec) = (126.189, -7.957) deg = (08 24 45, -07 57 25) (J2000) 
(R.A., Dec) = (126.475, -8.145) deg = (08 25 53, -08 08 41) (J2000) 
(R.A., Dec) = (127.569, -6.507) deg = (08 30 16, -06 30 25) (J2000) 
(R.A., Dec) = (127.284, -6.319) deg = (08 29 08, -06 19 08) (J2000) 
The X-ray flux averaged over the scan was 203 +- 29 mCrab
(4.0-10.0keV, 1 sigma error).
There was no significant excess flux in the previous transit at 01:49 UT 
with an upper limit of 20 mCrab.

N. Ito, R. Hosokawa, M. Niwano, K. L. Murata, Y. Imai, Y. Takamatsu,
R. Yamaguchi, R. Noto, S. Sato, M. Takaku, Y. Yatsu, and N. Kawai
(Tokyo Tech) report on behalf of the MITSuME collaboration:

We performed tiling observations for the MAXI/GSC error region of GRB
211225A, MAXI J0827-071 (T. Kurihara et al. GCN Circular #31303
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 , V.
Lipunov et al. GCN Circular #31304
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 ) with an optical tri-color (g', Rc,
and Ic) camera attached to the MITSuME 50 cm telescope Akeno.

The follow up observation started at 2021-12-25 12:43:40 UT, 9.4 hours
after the trigger.

Our tiling observation covered almost ~90% of the MAXI/GSC elliptical
error region (GCN Circular #31303
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 ). Comparing our co-added images with
the GSC2.3 catalog, we found no optical transients in the observed
region. The 5-sigma upper limits of the co-added images are listed
below.

Field num. | center position (R.A., Dec.) | size of field
(R.A.(arcmin) x Dec.(arcmin)) | 5-sigma upper limit | MID-UT (UT) |
exposure time (sec)
--------------------------------------------------------------------------------------------------------------------------
Field 00 | 8:27:04, -7:19:28 | 25.1 x 24.8 | g'>18.1, Rc>18.4, Ic>17.9
| 2021-12-25 15:30:02 | 1260
Field 01 | 8:28:39, -7:19:27 | 26.1 x 25.1 | g'>18.1, Rc>18.4, Ic>17.6
| 2021-12-25 15:36:28 | 1380
Field 02 | 8:27:01, -6:54:21 | 26.3 x 24.0 | g'>18.3, Rc>18.7 |
2021-12-25 16:31:42 | 2400
Field 03 | 8:28:37, -6:54:57 | 26.3 x 24.6 | g'>18.0, Rc>18.4, Ic>17.8
| 2021-12-25 16:48:59 | 1620
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We used 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).