GRB 151231A

S. Golenetskii, R.Aptekar, D. Frederiks, P. Oleynik, M. Ulanov,
D. Svinkin, A. Tsvetkova, A. Lysenko, A. Kozlova and T. Cline
on behalf of the Konus-Wind team, report:

The long-duration, intense GRB 151231A
(Fermi GBM observation: Veres, C Meegan & von Kienlin, GCN 18787
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 ;
CALET Gamma-Ray Burst Monitor detection: Moriyama et al., GCN18810
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 ;
IPN triangulation: Hurley et al., GCN 18855
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 )
triggered Konus-Wind at T0=38267.351 s UT (10:37:47.351).

The burst light curve shows two emission episodes
with a total duration of ~91 s.
The emission is seen up to ~15 MeV.

As observed by Konus-Wind, the burst
had a fluence of 9.03(-0.64,+0.65)x10^-5 erg/cm2,
and a 64-ms peak energy flux, measured from T0+5.952 s,
of 1.27(-0.19,+0.19)x10^-5 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The time-integrated spectrum of the burst
(measured from T0 to T0+77.056 s)
is best fit in the 20 keV - 15 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -0.81(-0.09,+0.10),
the high energy photon index beta = -2.57(-0.21,+0.14),
the peak energy Ep = 163(-10,+11) keV,
chi2 = 95/96 dof.

The spectrum near the peak count rate
(measured from T0 to T0+7.424 s)
is best fit in the 20 keV - 3 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -1.07(-0.07,+0.07),
the high energy photon index beta = -2.40(-0.19,+0.13),
the peak energy Ep = 216(-20,+22) keV,
chi2 = 61/63 dof.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB151231_T38267/

All the quoted errors are at the 90% confidence level.
All the quoted values are preliminary.

K. Hurley, on behalf of the IPN,

I. G. Mitrofanov, D. Golovin, M. L. Litvak, and A. B. Sanin,
on behalf of the HEND-Odyssey GRB team,

S. Golenetskii, R. Aptekar, D. Frederiks, D. Svinkin,
and T. Cline on behalf of the Konus-Wind team,

V. Connaughton, M. S. Briggs, C. Meegan, V. Pelassa,
and A. Goldstein, on behalf of the Fermi GBM team,

S. Barthelmy, J. Cummings, N. Gehrels, H. Krimm, and D. Palmer,
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 151231A was detected by Fermi (GBM: Veres, et al., 
GCN Circ. 18787
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 ), Konus-Wind, Swift (BAT), and Mars-Odyssey (HEND), at 
about 38267 s UT (10:37:47). 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:
    69.528 (04h 38m 07s) -61.010 (-61d 00' 36")
  Corners:
    70.843 (04h 43m 22s) -60.244 (-60d 14' 37")
    71.191 (04h 44m 46s) -60.345 (-60d 20' 41")
    68.143 (04h 32m 34s) -61.761 (-61d 45' 40")
    67.789 (04h 31m 09s) -61.652 (-61d 39' 07")
  ---------------------------------------------
The error box area is 1425 sq. arcmin, and its maximum
dimension is 2.1 deg (the minimum one is 11.6 arcmin).
The Sun distance was 91 deg.

This box may be improved.

A triangulation map is posted at
http://www.ioffe.ru/LEA/GRBs/GRB151231_T38267/IPN/

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

M. Moriyama, A. Yoshida, T. Sakamoto, I. Takahashi, Y. Kawakubo, K. Senuma, 
Y. Yamada (AGU), K. Yamaoka (Nagoya U), S. Nakahira (JAXA), 
Y. Asaoka, S. Ozawa, S. Torii (Waseda U), Y. Shimizu, T. Tamura (Kanagawa U), 
M. L. Cherry (LSU), S. Ricciarini (U of Florence), P. S. Marrocchesi (U of Siena) 
and the CALET collaboration:

The long-duration GRB 151231A (Veres et al. GCN Circ. 18787
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 ) triggered the CALET 
Gamma-Ray Burst Monitor (CGBM) at 10:37:53.15 UT on 31 December 2015.  The 
clear burst signal was detected by all instruments.  

The light curve of the SGM shows two main peaks.  The first peak starts at T0-2 sec, 
peaks at T0 and ends at T0+5 sec.  The second peak starts at T0+50 sec, peaks at 
T0+60 sec and ends at T0+70 sec.  The T90 duration measured by the SGM data is 
66.0 +- 0.7 sec (40-1000 keV).  

The CGBM data used in this analysis are provided by the Waseda CALET Operation Center 
located at the Waseda University.

P Veres (UAH), C Meegan (UAH) and A. von Kienlin (MPE)
report on behalf of the Fermi GBM Team:

"At 10:37:47.52 UT on 31 December 2015, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 151231A (trigger 473251071 / 151231443).

The on-ground calculated location, using the GBM trigger
data, is RA = 65.6, DEC = -61.5 (J2000 degrees,
equivalent to 04 h 23 m, -61 d 32 '), with an uncertainty
of 1 degrees (radius, 1-sigma containment,
statistical only; there is additionally a systematic
error which we have characterized as a core-plus-tail model, with 90% of
GRBs having a 3.7 deg error and a small tail suffering a larger than 10 deg
systematic error. [Connaughton et al. 2015, ApJS, 216, 32] ).

The angle from the Fermi LAT boresight at the GBM trigger time is 157 degrees.

The GBM light curve shows two well separated peaks
with a duration (T90) of about 71 s (50-300 keV).
The time-averaged spectrum from T0-2.3 s to T0+85 s is
best fit by a Band function with Epeak = 198 +/- 7 keV,
alpha = -1.00 +/- 0.04, and beta = -2.61 +/- 0.09

The event fluence (10-1000 keV) in this time interval is
(9.7 +/- 0.1)E-5 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+5.3 s in the 10-1000 keV band
is 46.9 +/- 1.5 ph/s/cm^2.


The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog."