GRB 231222B

N. Di Lalla (Stanford University), F. Longo (University and INFN, Trieste), A. Holzmann (DF, University of Buenos Aires, Argentina) report on behalf of the Fermi-LAT Collaboration:

On December 22, 2023 Fermi-LAT detected high-energy emission from GRB 231222B, which was also detected by Fermi-GBM (trigger 724922767 / 231222310).

The best LAT on-ground location is found to be

RA, Dec = 251.35, 19.55 (degrees, J2000)

with an error radius of 0.3 deg (90% containment, statistical error only). This was 19 deg from the LAT boresight at the time of the GBM trigger:

T0 = 07:26:02.19 UT.

The data from the Fermi-LAT show a significant increase in the event rate after the GBM trigger that is spatially correlated with the GBM emission with high significance. The photon flux above 100 MeV in the time interval**0-600 s after the GBM trigger is (1.9 +/- 0.7) E-6**ph/cm2/s.

The estimated photon index above 100 MeV is -1.7 +/- 0.3. The highest-energy photon is a 3.6 GeV event which is observed 362 seconds after the GBM trigger.

A Swift ToO has been approved for this burst.

The Fermi-LAT point of contact for this burst is Makoto Arimoto (arimoto@se.kanazawa-u.ac.jp <mailto:arimoto@se.kanazawa-u.ac.jp>)

The Fermi-LAT is a pair conversion telescope designed to cover the energy band from 20 MeV to greater than 300 GeV. It is the product of an international collaboration between NASA and DOE in the U.S. and many scientific institutions across France, Italy, Japan and Sweden.

P. Veres (UAH), V. Sharma (NASA-GSFC/UMBC) and R. Hamburg (CNRS/IN2P3) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:

"At 07:26:02.19 UT on 22 December 2023, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 231222B (trigger 724922767/231222310).
The GRB was also detected by Fermi LAT (Di Lalla et al, GCN 35416
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 ).

The angle from the Fermi LAT boresight is 19 degrees.

The GBM light curve consists of two widely separated pulses with a duration (T90)
of about 121 s (50-300 keV). The time-averaged spectrum
from T0-5.1 to T0+126.0 s is best fit by
a simple power law function with index -1.55 +/- 0.04.

The event fluence (10-1000 keV) in this time interval is
(5.7 +/- 0.6)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+1.7 s in the 10-1000 keV band is 6.0 +/- 0.3 ph/s/cm^2.


A power law function with an exponential
high-energy cutoff fits the spectrum equally well.
The power law index is -1.09 +/- 0.16 and
the cutoff energy, parameterized as Epeak, is 257 +/- 67 keV.


The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog:
https://heasarc.gsfc.nasa.gov/W3Browse/fermi/fermigbrst.html

For Fermi GBM data and info, please visit the official Fermi GBM Support Page:
https://fermi.gsfc.nasa.gov/ssc/data/access/gbm/"

M. Ferro (INAF-OAB), J.A. Kennea (PSU), A. Tohuvavohu (U. Toronto),
D.N. Burrows (PSU), J.P. Osborne (U. Leicester), K.L. Page (U.
Leicester), A.P. Beardmore (U. Leicester), R. Brivio (INAF-OAB), A.
D'Ai (INAF-IASFPA) and P.A. Evans (U. Leicester) reports on behalf of
the Swift-XRT team:

Swift-XRT has performed follow-up observations of the
Fermi/LAT-detected burst GRB 231222B in a series of observations tiled
on the sky. We point out that the burst was improperly named GRB
231222A in GCN Circ 35415
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 , where the tiling was announced. The total
exposure time is 4.1 ks, distributed over 4 tiles; the maximum exposure
at a single sky location was 2.3 ks. The data were collected between
T0+43.2 ks and T0+54.8 ks, and are entirely in Photon Counting (PC)
mode. 

Three uncatalogued X-ray sources are detected, of which one ("Source
1") is believed to be the afterglow. Using 1211 s of PC mode data and 1
UVOT image, we find an enhanced XRT position (using the XRT-UVOT
alignment and matching UVOT field sources to the USNO-B1 catalogue):
RA, Dec = 251.24744, +19.62084 which is equivalent to:

RA (J2000): 16h 44m 59.39s
Dec(J2000): +19d 37' 15.0"

with an uncertainty of 3.3 arcsec (radius, 90% confidence). This
position is 9.3 arcmin from the Fermi/LAT position.  The source has a
mean count rate of 6.0e-02 ct/sec; we cannot determine at the present
time whether it is fading.

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.65 (+0.49, -0.29). The
best-fitting absorption column is  consistent with the Galactic value
of 7.1 x 10^20 cm^-2 (Willingale et al. 2013). The counts to observed
(unabsorbed) 0.3-10 keV flux conversion factor deduced from this
spectrum  is 3.4 x 10^-11 (3.8 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     7 (+/-14) x 10^20 cm^-2
Galactic foreground: 7.1 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.65 (+0.49, -0.29)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00021639.
The results of the full analysis of the tiled XRT observations are
available at https://www.swift.ac.uk/xrt_products/TILED_GRB00118.

This circular is an official product of the Swift-XRT team.

M. Dafcikova, J. Ripa (Masaryk U.), A. Pal (Konkoly Observatory), N. Werner (Masaryk U.), M. Ohno, H. Takahashi (Hiroshima U.), L. Meszaros, B. Csak (Konkoly Observatory), N. Husarikova, F. Munz , M. Topinka, M. Kolar, L. Szakszonova, J.-P. Breuer, F. Hroch (Masaryk U.), T. Urbanec, M. Kasal,  A. Povalac (Brno U. of Technology), J. Hudec, J. Kapus, M. Frajt (Spacemanic s.r.o), R. Laszlo, M. Koleda (Needronix s.r.o), M. Smelko, P. Hanak, P. Lipovsky (Technical U. of Kosice), G. Galgoczi (Wigner Research Center/Eotvos U.), Y. Uchida, H. Poon, H. Matake (Hiroshima U.), N. Uchida (ISAS/JAXA), T. Bozoki (Eotvos U.), G. Dalya (Eotvos U.), T. Enoto (Kyoto U.), Zs. Frei (Eotvos U.), G. Friss (Eotvos U.), Y. Fukazawa, K. Hirose (Hiroshima U.), S. Hisadomi (Nagoya U.), Y. Ichinohe (Rikkyo U.), K. Kapas (Eotvos U.), L. L. Kiss (Konkoly Observatory),  T. Mizuno (Hiroshima U.), K. Nakazawa (Nagoya U.), H. Odaka (Univ of Tokyo), J. Takatsy (Eotvos U.), K. Torigoe (Hiroshima U.), N. Kogiso, M. Yoneyama (Osaka Metropolitan U.), M. Moritaki (U. Tokyo), T. Kano (U. Michigan) -- the GRBAlpha collaboration.

The long-duration GRB 231222B (Fermi/GBM detection: GCN 35418
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 ; Fermi/LAT detection: GCN 35416
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 ; INTEGRAL/SPI-ACS peak detection at 2023-12-22 ~07:26:03 UT) was observed by the GRBAlpha 1U CubeSat (Pal et al. 2023, A&A, 677, 40; arXiv:2302.10048
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 ).

The detection was confirmed at the peak time 2023-12-22 07:26:04 UTC. The T90 duration measured by GRBAlpha is 5 s and the overall significance during T90 reaches 6.2 sigma.

The light curve obtained by GRBAlpha is available here: https://grbalpha.konkoly.hu/static/share/GRB231222B_GCN.pdf

All GRBAlpha detections are listed at: https://monoceros.physics.muni.cz/hea/GRBAlpha/ 
GRBAlpha, launched on 2021 March 22, is a demonstration mission for a future CubeSat constellation (Werner et al. Proc. SPIE 2018). The detector of GRBAlpha consists of a 75 x 75 x 5 mm3 CsI scintillator read out by a SiPM array, covering the energy range from ~50 keV to ~1000 keV. To increase the duty cycle and the downlink rate, the upgrade of the on-board data acquisition software stack is in progress. The ground segment is also supported by the radio amateur community and it takes advantage of the SatNOGS network for increased data downlink volume.