GRB 210927B

J. C. Liu, S. L. Xiong, X. Y. Zhao, Y. Huang, X. Y. Song, C. Cai, S. Xiao,
S. L. Xie, Y. Q. Zhang, Y. Zhao, Z. W. Guo, C. Zheng, W. C. Xue, C. Y. Li,
C. W. Wang, Q. B. Yi, B. X. Zhang, P. Zhang, W. X. Peng, R. Qiao, D. Y. Guo,
X. B. Li, X. Ma, P. Wang, J. Wang, Z. Zhang, S. J. Zheng, W. Chen, J. J. He, 
G. Y. Zhao, Y. Q. Du, H. Wu, J. Liang, Q. Luo, X. L. Zhang, L. M. Song, 
F. J. Lu, S. N. Zhang (IHEP), report on behalf of GECAM team:

During the commissioning phase, GECAM-B was triggered in-flight by a long
burst, GRB 210927B, at 2021-09-27T23:54:45.600 UTC (denoted as T0)���
which was also observed by Fermi/GBM(trigger ID: 654479691)

Its alert data was promptly downlinked to the ground through the short
message service of BeiDou Navigation Satellite System (BDS). The time
latency of the first BeiDou message relative to the trigger time is about
1 minute.

According to the BDS alert data, this burst mainly consists of multiple
pulses with duration of about 120 s. 

The GECAM light curve could be found here:
http://twiki.ihep.ac.cn/pub/GECAM/GRBList/86486085.60000002energys.png

GECAM-B localized this burst to the following position (J2000): 
Ra: 276.4 deg 
Dec: 76.0 deg
Err: 3.0 deg (1-sigma, statistical only)
The current systematic error of location is estimated to be several degrees 
which could be minimized by the ongoing calibration.

The GECAM preliminary location could be found here: 
http://twiki.ihep.ac.cn/pub/GECAM/GRBList/gecamb_skymap_bdm_86486085_V01.png

This location is consistent with Fermi/GBM position within the error.

Please note that all GECAM results here are preliminary. The final analysis
will be published in journal papers or GECAM online catalog.

Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor
(GECAM) mission consists of two small satellites (GECAM-A and GECAM-B) in
Low Earth Orbit (600 km, 29 deg), launched on Dec 10, 2020 (Beijing Time),
which was funded by the Chinese Academy of Sciences (CAS).

A.S. Kozyrev, D.V. Golovin, M.L. Litvak, I.G. Mitrofanov, and A.B. Sanin
on behalf of the MGNS/BepiColombo and HEND/Mars Odyssey teams,

J. Benkhoff on behalf of the BepiColombo team,

K. Hurley, on behalf of the IPN,

D. Svinkin, S. Golenetskii, D. Frederiks, A. Ridnaia,
A. Lysenko, and T. Cline on behalf of the Konus-Wind team,

A. Goldstein, M. S. Briggs, and C. Wilson-Hodge
on behalf of the Fermi GBM team,

A. von Kienlin, X. Zhang, A. Rau, V. Savchenko, E. Bozzo,
and C. Ferrigno, on behalf of the INTEGRAL SPI-ACS GRB team,

S. Barthelmy, J. Cummings, H. Krimm, D. Palmer, and A. Tohuvavohu
on behalf of the Swift-BAT team, and

W. Boynton, C. Fellows, K. Harshman, H. Enos, and R. Starr,
on behalf of the GRS-Odyssey GRB team, report:

The long-duration GRB 210927B (GECAM-B detection: GCN circ. 30910
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 )
was detected by BepiColombo (MGNS), in the vicinity of Mercury
(see introductory GCN Circ. 30949
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 ); Mars-Odyssey (HEND);
GECAM-B; Fermi (GBM trigger 654479691); Konus-Wind; INTEGRAL (SPI-ACS);
and Swift (BAT) at about 86086 s UT (23:54:46).
The burst was outside the coded field of view of the BAT.

The burst light curves in the energy range 30 keV-1.5 MeV (HEND)
and 250-450 keV (MGNS) exhibit a multi-peaked pulse structure
with a total duration of ~25 s.

We have triangulated GRB 210927B to the following 3 sigma annuli:
---------------------------------------------------------
          R.A.(J2000)  Dec.(J2000)    R      +dR     -dR
                deg      deg         deg     deg     deg
---------------------------------------------------------
MGNS-HEND     2.101    -1.674      94.136  +0.034 -0.079
Konus-HEND  187.856    -2.595      88.396  +0.017 -0.017
Konus-GBM   161.442     7.403      85.952  +0.562 -0.562
---------------------------------------------------------

The MGNS-HEND and Konus-HEND annuli intersect to form two error
boxes one of which is inconsistent with the Konus-GBM annulus,
the KW ecliptic latitude response, and the Fermi (GBM) final position.

The correct preliminary, 3 sigma error box is:
  ---------------------------------------------
   RA(2000), deg                 Dec(2000), deg
  ---------------------------------------------
  Center:
   263.021 (17h 32m 05s) +73.775 (+73d 46' 32")
  Corners:
   263.554 (17h 34m 13s) +73.267 (+73d 15' 59")
   262.667 (17h 30m 40s) +74.201 (+74d 12' 03")
   262.825 (17h 31m 18s) +73.913 (+73d 54' 45")
   263.687 (17h 34m 45s) +72.979 (+72d 58' 46")
  ---------------------------------------------
The error box area is 49 sq. arcmin, and its maximum
dimension is 1.26 deg (the minimum one is 2.0 arcmin).
The Sun distance was 89 deg.

This box may be improved.

A triangulation map is posted at:
http://l503.iki.rssi.ru/owncloud/index.php/s/nFbsu69DWdxQvd2