GRB 221226B

The Fermi GBM team reports the detection of a likely LONG GRB

At 22:41:20 UT on 26 Dec 2022, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 221226B (trigger 693787285.418586 / 221226945).

The on-ground calculated location, using the Fermi GBM trigger data, is RA = 16.3, Dec = -53.0 (J2000 degrees, equivalent to J2000 01h 05m, -53d 00'), with a statistical uncertainty of 9.8 degrees.

The angle from the Fermi LAT boresight is 46.0 degrees.

The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2022/bn221226945/quicklook/glg_skymap_all_bn221226945.png

The HEALPix FITS file, including the estimated localization systematic, can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2022/bn221226945/quicklook/glg_healpix_all_bn221226945.fit

The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2022/bn221226945/quicklook/glg_lc_medres34_bn221226945.gif

S. B. Cenko (GSFC), J.D. Gropp (PSU), J. A. Kennea (PSU),
H. A. Krimm (NSF), S. Laha (GSFC/UMBC), A. Y. Lien (U Tampa),
F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester),
D. M. Palmer (LANL) and T. M. Parsotan (GSFC/UMBC/CRESSTII) report on
behalf of the Neil Gehrels Swift Observatory Team:

At 22:41:20 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 221226B (trigger=1145959).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 22.917, -41.554 which is 
   RA(J2000) = 01h 31m 40s
   Dec(J2000) = -41d 33' 13"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a multi-peaked
structure with a duration of about 10 sec.  The peak count rate
was ~2200 counts/sec (15-350 keV), at ~1 sec after the trigger. 

The XRT began observing the field at 22:42:32.4 UT, 72.4 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source located at RA, Dec 22.90890, -41.52648 which
is equivalent to:
   RA(J2000)  = 01h 31m 38.14s
   Dec(J2000) = -41d 31' 35.3"
with an uncertainty of 3.5 arcseconds (radius, 90% containment). This
location is 101 arcseconds from the BAT onboard position, within the
BAT error circle. This position may be improved as more data are
received; the latest position is available at
https://www.swift.ac.uk/sper. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 1.69
x 10^20 cm^-2 (Willingale et al. 2013). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 75 seconds after the BAT trigger. No credible afterglow candidate has
been found in the initial data products. The 2.7'x2.7' sub-image covers none of
the XRT error circle. The 8'x8' region for the list of sources generated
on-board covers 100% of the XRT error circle. The list of sources is typically
complete to about 18 mag. No correction has been made for the expected
extinction corresponding to E(B-V) of 0.017. 

Burst Advocate for this burst is S. B. Cenko (brad.cenko AT nasa.gov). 
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/)

M.R. Goad, J.P. Osborne, A.P. Beardmore and P.A. Evans (U. Leicester) 
report on behalf of the Swift-XRT team.

Using 1173 s of XRT Photon Counting mode data and 2 UVOT
images for GRB 221226B, we find an astrometrically corrected X-ray
position (using the XRT-UVOT alignment and matching UVOT field sources
to the USNO-B1 catalogue): RA, Dec = 22.90972, -41.52669 which is equivalent
to:

RA (J2000): 01h 31m 38.33s
Dec (J2000): -41d 31' 36.1"

with an uncertainty of 2.6 arcsec (radius, 90% confidence).

This position may be improved as more data are received. The latest
position can be viewed at http://www.swift.ac.uk/xrt_positions. Position
enhancement is described by Goad et al. (2007, A&A, 476, 1401) and Evans
et al. (2009, MNRAS, 397, 1177).

This circular was automatically generated, and is an official product of the
Swift-XRT team.

D. Xu (NAOC), A. Chrimes (Radboud Univ.), B. Schneider (MIT),  L. Izzo 
(DARK/NBI), A. Saccardi (GEPI, Observatoire de Paris), Z.P. Zhu (NAOC, 
HUST), N.R. Tanvir (Univ. Leicester) report on behalf of the Stargate 
collaboration:

We observed the field of GRB 221226B detected by Swift (Cenko et al., 
GCN 33108
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 ) and Fermi (Fermi GBM team, GCN 33107
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 ) using the ESO VLT UT3 
(Melipal) equipped with the X-shooter spectrograph. Our spectra cover 
the wavelength range 3000-21000 AA, and consist of 4 exposures by 1200 s 
each.

The observations started at 01:10:06 UT on 2022-12-27, i.e., ~2.48 hr 
after BAT trigger. The optical afterglow of the burst is detected at 
coordinates

R.A.(J2000) = 01:31:38.036
Dec.(J2000) = - 41:31:37.25

with an uncertainty of ~0.3 arcsec and r = 22.0 +/- 0.1 mag, calibrated 
with Legacy Survey nearby stars.

A broad absorption feature is visible around 4490 AA, which we identify 
as being due to H I. From detection of multiple absorption features, 
which we interpret as being due to S II, Si II, O I, C II, Si IV, Al 
III, NV, and emission features due to [O II] and Hbeta,  we infer a 
common redshift of z = 2.694. We conclude this is the redshift of the burst.

Note that at the optical afterglow position, the Legacy Survey presents 
an extended source with r = 23.82 mag, which is likely the host galaxy 
of the burst, being consistent with the the detection of emission 
features in the X-shooter spectra.

We acknowledge excellent support from the ESO observing staff in 
Paranal, in particular Pascale Hibon and Ditte Slumstrup.

M. Capalbi (INAF-IASFPA), M. Perri (SSDC & INAF-OAR), J.A. Kennea
(PSU), A. Tohuvavohu (U. Toronto), D.N. Burrows (PSU), P.A. Evans (U.
Leicester), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), V.
D'Elia (SSDC & INAF-OAR) and S.B. Cenko report on behalf of the
Swift-XRT team:

We have analysed 6.2 ks of XRT data for GRB 221226B (Cenko et al. GCN
Circ. 33108
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 ), from 56 s to 40.4 ks after the  BAT trigger. The data
comprise 14 s in Windowed Timing (WT) mode (the first 9 s were taken
while Swift was slewing) with the remainder in Photon Counting (PC)
mode. The enhanced XRT position for this burst was given by Goad et al.
(GCN Circ. 33109
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 ).

The light curve can be modelled with a power-law decay with a decay
index of alpha=1.58 (+0.13, -0.12).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.5 (+0.4, -0.3). The
best-fitting absorption column is  7 (+23, -7) x 10^21 cm^-2, at a
redshift of 2.694, in addition to the Galactic value of 1.7 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 4.3 x
10^-11 (4.6 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Galactic foreground: 1.7 x 10^20 cm^-2
Intrinsic column:    7 (+23, -7) x 10^21 cm^-2 at z=2.694
Photon index:	     1.5 (+0.4, -0.3)

If the light curve continues to decay with a power-law decay index of
1.58, the count rate at T+24 hours will be 6.9 x 10^-5 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.0 x
10^-15 (3.2 x 10^-15) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01145959.

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

S. Lesage (UAH) and C. Meegan (UAH) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:

"At 22:41:20 UT on 26 December 2022, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 221226B (trigger 693787285/221226945),
which was also detected by Swift BAT (S. B. Cenko et al. 2022, GCN 33108
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 )
and has a measured redshift from the VLT/X-Shooter (GCN 33110
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 ). The Fermi
GBM Final Real-time Localization (GCN 33107
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 ) is consistent with the Swift
BAT position.

The angle from the Fermi LAT boresight is 58 degrees.

The GBM light curve consists of a single emission episode with a duration
(T90)
of about 5 s (50-300 keV). The time-averaged spectrum
from T0+0.002 to T0+2.592 s is best fit by
a power law function with an exponential high-energy cutoff.
The power law index is 0.2 +/- 0.2 and the cutoff energy,
parameterized as Epeak, is 104 +/- 8 keV.

The event fluence (10-1000 keV) in this time interval is
(7.8 +/- 0.5)E-07 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+0.3 s in the 10-1000 keV band is 4.1 +/- 0.2 ph/s/cm^2.

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/"

N. Klingler (UMBC/NASA-GSFC/CRESST II) reports on behalf of the Swift/UVOT team:

The Swift-UVOT began settled observations of the field of GRB 221226B
76 s after the BAT trigger (Cenko et al., GCN Circ. 33108
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 ).  A source
consistent with the XRT position (Goad et al., GCN Circ. 33109
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 ) and consistent
with the  optical  counterpart reported by VLT/X-shooter
(Xu et al., GCN Circ. 33110
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 ) is detected in the initial UVOT exposures.

Preliminary detections and 3-sigma upper limits using the UVOT
photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the early exposures are:

Filter         T_start(s)   T_stop(s)      Exp(s)           Mag

white               76          226          147         20.54 +/- 0.22
v                 4408         4608          197        >19.3
b                 3793         3993          197        >20.0
u                  288         5132          278        >20.1
w1                4818         5018          197        >20.0
m2                4613         4813          197        >19.5
w2                4204         4404          197        >20.0

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.017 in the direction of the burst
(Schlegel et al. 1998).

S. Laha (GSFC/UMBC), S. D. Barthelmy (GSFC), S. B. Cenko (GSFC),
H. A. Krimm (NSF), A. Y. Lien (U Tampa), C. B. Markwardt (GSFC),
D. M. Palmer (LANL), T. Parsotan (GSFC/UMBC), T. Sakamoto (AGU),
M. Stamatikos (OSU) (i.e. the Swift-BAT team):

Using the data set from T-240 to T+963 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 221226B (trigger #1145959)
(Cenko, et al., GCN Circ. 33108
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 ).  The BAT ground-calculated position is
RA, Dec = 22.903, -41.515 deg which is
  RA(J2000)  =  01h 31m 36.7s
   Dec(J2000) = -41d 30' 52.5"
with an uncertainty of 1.2 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 97%.

The mask-weighted light curve shows a pulse of approximately 4 seconds
total duration and some substructure.   T90 (15-350 keV) is 3.44 +- 0.57 sec
(estimated error including systematics).

The time-averaged spectrum from T+0.22 to T+4.28 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.16 +- 0.13.  The fluence in the 15-150 keV band is 5.4 +- 0.5 x 10^-7 erg/cm2.
The 1-sec peak photon flux measured from T+0.24 sec in the 15-150 keV band
is 2.6 +- 0.3 ph/cm2/sec.  All the quoted errors are at the 90% confidence
level.
The results of the batgrbproduct analysis are available at
http://gcn.gsfc.nasa.gov/notices_s/1145959/BA/

R. Strausbaugh (University of Minnesota), A. Cucchiara (NASA) report on
behalf of a larger collaboration:

We observed the GRB 221226B (Cenko et al., GCN 33108
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 ) field with the LCOGT
1-meter Sinistro instrument at the South African Astronomical Observatory
site, on December 27, from 19:40 to 20:07 UT (corresponding to 20.98 to
21.47 hours from the GRB trigger time) with the SDSS r and i filters.

We performed a series of 3x300s exposures in each band.  We do not detect
any new sources in the XRT error region, in either band.

The following upper limits are calculated using the SkyMapper catalog as
reference:

r > 20.75
i > 20.16

These magnitudes are not corrected for galactic extinction.