GRB 210531A

B. Biltzinger, F. Kunzweiler, F. Berlato, J. Burgess & J. Greiner (all MPE Garching) report:

The public trigdat data of the Fermi Gamma-ray Burst Monitor (GBM) trigger
644155206 at 12:00:01 on 31 May 2021 were automatically fitted for spectrum
and sky location with BALROG (Burgess et al. 2018, MNRAS 476, 1427;
Berlato et al. 2019, ApJ 873, 60).

The best-fit position (1 sigma statistical errors) is:
RA(2000.0) = 140.9+/-0.5 deg
Decl.(2000.0) = 6.1+/-0.7 deg
We estimate an additional systematic error of 2 deg.

Further details are available at:
https://grb.mpe.mpg.de/grb/GRB210531500/

The Healpix map can be downloaded from:
https://grb.mpe.mpg.de/grb/GRB210531500/healpix

The location parameters are available as JSON at:
https://grb.mpe.mpg.de/grb/GRB210531500/json

A. Ursi (INAF/IAPS), F. Longo (Univ. Trieste and INFN Trieste), M. Tavani
(INAF/IAPS, and Univ. Roma Tor Vergata), A. Argan, M. Cardillo, C.
Casentini, Y. Evangelista, G. Piano (INAF/IAPS), F. Lucarelli, C. Pittori,
F. Verrecchia (SSDC, and INAF/OAR), A. Bulgarelli, A. Di Piano,  V.
Fioretti, F. Fuschino, N. Parmiggiani (INAF/OAS-Bologna), M. Marisaldi
(INAF/OAS-Bologna, and Bergen University), M. Pilia, A. Trois
(INAF/OA-Cagliari), I. Donnarumma (ASI), A. Giuliani (INAF/IASF-Mi), report
on behalf of the AGILE Team:

The AGILE satellite detected the long GRB 210531A at T0 = 2021-05-31
12:00:02.66 +/- 0.01 s (UTC), reported by Fermi/GBM (Trigger no. 644155206)
and BALROG (GCN #30105
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 ).

The burst is clearly visible in the AGILE scientific ratemeters of the
SuperAGILE (SA; 20-60 keV), MiniCALorimeter (MCAL; 0.4-100 MeV), and
AntiCoincidence (AC; 50-200 keV) detectors. The event lasted about 35 s and
it released a total number of 9650 counts in the SA detector (above a
background rate of 90 Hz), 53900 counts in the MCAL detector (above a
background rate of 1400 Hz), and 151900 counts in the AC detector (above a
background rate of 3780 Hz). The AGILE ratemeter light curves can be found
at http://www.agilescienceapp.it/notices/GRB210531A_AGILE_RM.png .

The event also triggered a partial high time resolution MCAL data
acquisition.

Additional analysis of AGILE data is in progress. Automatic MCAL GRB alert
Notices can be found at: https://gcn.gsfc.nasa.gov/agile_mcal.html.

P. Sawant (IUCAA), G. Waratkar (IITB), A. Vibhute (IUCAA), S. Gupta
(IUCAA), V. Bhalerao (IITB), D. Bhattacharya (IUCAA), A. R. Rao
(IUCAA/TIFR), and S. Vadawale (PRL) report on behalf of the AstroSat
CZTI collaboration:

Analysis of AstroSat CZTI data with the CIFT framework (Sharma et al,
2020, arxiv:2011.07067) showed detection of a long GRB 210531A, which
was also detected by AGILE/MCAL (GCN #30106
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 ) and Fermi (GCN #30105
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 ).

The source was clearly detected in the 20-200 keV energy range. The
light curve showed multiple peaks of emission with the strongest peak at
2021-05-31 12:00:04.5 UT. The measured peak count rate associated with
the burst is 1948 (+77, -87) cts/s above the background in the combined
data of four quadrants, with a total of 13005 (+281, -332) cts. The
local mean background count rate was 532 (+3.0, -2.9) cts/s. Using
cumulative rates, we measure a T90 of 20 (+0.2, -0.2)s. 

It was also clearly detected in the CsI anticoincidence (Veto) detector
in the 100-500 keV energy range. The light curve showed multiple peaks
of emission with the strongest peak at 2021-05-31 12:00:04.9 UT. The
measured peak count rate is 2541 (+104, -114) cts/s above the background
in the combined Veto data of four quadrants, with a total of 16360
(+452, -552) cts. The local mean background count rate was 1729 (+5, -5)
cts/s. We measure a T90 of 19 (+1, -2) s from the cumulative Veto light
curve.

CZTI GRB detections are reported regularly on the payload site at
http://astrosat.iucaa.in/czti/?q=grb [1]. CZTI is built by a TIFR-led
consortium of institutes across India, including VSSC, ISAC, IUCAA, SAC
and PRL. The Indian Space Research Organisation funded, managed and
facilitated the project. 

Links:
------
[1] http://astrosat.iucaa.in/czti/?q=grb

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

K. Hurley, on behalf of the IPN,

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,
report:

The bright, long-duration GRB 210531A
(BALROG localization: Biltzinger et al., GCN Circ. 30105
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 ;
AGILE detection: Ursi et al., GCN Circ. 30106
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 ;
AstroSat-CZTI detection: Sawant et al., GCN Circ. 30108
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 )
was detected by Fermi (GBM trigger 644155206), Konus-Wind,
INTEGRAL (SPI-ACS), AGILE (MCAL, SA, and AC),
and AstroSat (CZTI) at about 43201 s UT (12:00:01).

We have triangulated it to a preliminary, 3 sigma error box
whose coordinates are:
---------------------------------------------
   RA(2000), deg                 Dec(2000), deg
  ---------------------------------------------
  Center:
   140.592 (09h 22m 22s)  +8.281 ( +8d 16' 50")
  Corners:
   141.277 (09h 25m 06s) +11.425 (+11d 25' 30")
   142.066 (09h 28m 16s) +10.564 (+10d 33' 51")
   139.800 (09h 19m 12s)  +5.254 ( +5d 15' 16")
   139.052 (09h 16m 12s)  +6.081 ( +6d 04' 51")
  ---------------------------------------------
The error box area is 5.9 sq. deg, and its maximum
dimension is 6.3 deg (the minimum one is 1 deg).
The Sun distance was 70 deg.

This box may be improved.

The IPN localization is consistent with, but reduces the area of, the
Fermi final position and BALROG (GCN Circ. 30105
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 ) localizations.

A triangulation map is posted at
http://www.ioffe.ru/LEA/GRBs/GRB210531_T43198/IPN/

The Konus-Wind time history and spectrum will be given in a forthcoming
GCN Circular.