GRB 211226A

A. Ursi (INAF/IAPS), F. Verrecchia, C. Pittori (SSDC, and INAF/OAR), F.
Longo (Univ. Trieste and INFN Trieste), M. Tavani (INAF/IAPS, and Univ.
Roma Tor Vergata), A. Argan, M. Cardillo, C. Casentini, Y. Evangelista, L.
Foffano, E. Menegoni, G. Piano (INAF/IAPS), F. Lucarelli (SSDC, and
INAF/OAR), A. Bulgarelli, 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 Mini-CALorimeter (MCAL) detected a long burst at T0 = 2021-12-26
05:25:41.00 +/- 0.01 s (UTC) (INTEGRAL SPI-ACS trigger Tri,Sub: 9576,0).

The event lasted about 20 s and released a total number of 2736 counts in
the detector (in the 0.4-100 MeV energy range), above an average background
rate of 613 Hz. The MCAL light curve shows several peaks and can be found
at http://www.agilescienceapp.it/notices/GRB211226A_AGILE_MCAL.png .

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 released a total
number of 492 counts in the SA detector (above a background rate of 115
Hz), 5996 counts in the MCAL detector (above a background rate of 1290 Hz),
and 16770 counts in the AC detector (above a background rate of 3530 Hz).
The AGILE ratemeter light curves can be found at
http://www.agilescienceapp.it/notices/GRB211226A_AGILE_RM.png .

The AGILE-MCAL detector is a CsI detector with a 4 pi FoV, sensitive in the
energy range 0.4-100 MeV. 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.

Aaron Tohuvavohu (U Toronto), James DeLaunay (UAlabama), Gayathri
Raman (PSU), Jamie A. Kennea (PSU), report:

Swift/BAT did not localize GRB 211226A (T0: 2021-12-26T05:25:41 UTC,
INTEGRAL/SPI-ACS trig #9576, AGILE GCN 31309
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 ).

The INTEGRAL notice, distributed in near real-time, triggered the
Swift Mission Operations Center operated Gamma-ray Urgent Archiver for
Novel Opportunities (GUANO; Tohuvavohu et al. 2020, ApJ, 900, 1).

Upon trigger by this notice, GUANO sent a command to the Swift Burst
Alert Telescope (BAT) to save 90 seconds of BAT event-mode data from
[-45,+45] seconds around the time of the burst. All the requested
event mode data was delivered to the ground.

The BAT likelihood search, NITRATES (DeLaunay + Tohuvavohu,
arXiv:2111.01769
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 ), detects the burst with a sqrt(TS) of 86.7 in a
8.192 s analysis time bin.
The duration of the burst in BAT is ~12 seconds.

NITRATES results strongly indicate a burst coming from outside the
coded FOV, with DeltaLLHOut of -256.

See Section 9.1 and Figure 20 in the NITRATES paper for brief
descriptions and interpretation of sqrt(TS), DeltaLLHPeak, and
DeltaLLHOut.

GUANO is a fully autonomous, extremely low latency, spacecraft
commanding pipeline designed for targeted recovery of BAT event mode
data around the times of compelling astrophysical events to enable
more sensitive GRB searches.

A live reporting of Swift/BAT event data recovered by GUANO can be
found at: https://www.swift.psu.edu/guano/

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,

D. Svinkin, D. Frederiks, A. Ridnaia, A. Lysenko,
and T. Cline on behalf of the Konus-Wind 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 211226A
(AGILE/MCAL detection: Ursi et al., GCN Circ. 31309
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 ;
Swift/BAT-GUANO detection: Tohuvavohu et al., GCN Circ. 31313
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 )
was detected by AGILE (MCAL), Swift (BAT), Konus-Wind,
INTEGRAL (SPI-ACS), Mars-Odyssey (HEND), and BepiColombo (MGNS)
at about 19540 s UT (05:25:40).
The burst was outside the coded field of view of the BAT.

We have triangulated it to a preliminary, 3 sigma error box
whose coordinates are:
  ---------------------------------------------
   RA(2000), deg                 Dec(2000), deg
  ---------------------------------------------
  Center:
   241.834 (16h 07m 20s) -31.406 (-31d 24' 20")
  Corners:
   242.222 (16h 08m 53s) -31.675 (-31d 40' 30")
   241.641 (16h 06m 34s) -31.445 (-31d 26' 41")
   241.450 (16h 05m 48s) -31.118 (-31d 07' 03")
   242.025 (16h 08m 06s) -31.364 (-31d 21' 50")
  ---------------------------------------------
The error box area is 424 sq. arcmin, and its maximum
dimension is 52 arcmin (the minimum one is 12 arcmin).
The Sun distance was 30 deg.

This box may be improved.

A triangulation map is posted at
http://www.ioffe.ru/LEA/GRBs/GRB211226_T19539/IPN