GRB 151225A

T. Sakamoto, Y. Kawakubo (AGU) and the CALET collaboration:

We would like to make the correction to the reported trigger 
time of the CALET Gamma-Ray Burst Monitor (CGBM) for GRB 151225A 
(Kawakubo et al., GCN Circ. 18724
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 ).  The correct trigger time is 
19:09:09.71 UT on 25 December 2015.  We apologize for any confusion.  

We would like to thank Dmitry Svinkin for pointing this out.

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 short-duration, intense GRB 151225A
(CALET detection: Kawakubo et al., GCN 18724
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 ;
IPN triangulation: Golenetskii et al., GCN 18836
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 )
triggered Konus-Wind at T0=68946.508 s UT (19:09:06.508).

The burst light curve shows a  multipeaked structure
with a total duration of ~1.1 s.
The emission is seen up to ~2 MeV.

As observed by Konus-Wind, the burst
had a fluence of 3.17(-0.85,+1.79)x10^-6 erg/cm2,
and a 16-ms peak energy flux, measured from T0+0.416 s,
of 2.22(-0.79,+1.46)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+8.448 s)
is best fit in the 20 keV - 2 MeV range
by a power law with exponential cutoff model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with  alpha = -1.13(-0.48,+0.74),
and Ep = 360(-153,+741) keV (chi2 = 56/62 dof).
Fitting by the GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index:
beta < -1.7 (chi2 = 56/61 dof).

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB151225_T68946/

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

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

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,

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, 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 short-duration, hard-spectrum GRB 151225A (Kawakubo et al., GCN 
Circ. 18724
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 ) has been detected by CALET (CGBM), Konus-Wind, Swift (BAT), 
INTEGRAL (SPI-ACS), and Mars-Odyssey (HEND), so far, at about 68950 s UT 
(19:09:10). 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:
   227.901 (15h 11m 36s) -18.302 (-18d 18' 07")
  Corners:
   228.654 (15h 14m 37s) -16.789 (-16d 47' 21")
   228.463 (15h 13m 51s) -16.574 (-16d 34' 25")
   227.176 (15h 08m 42s) -19.869 (-19d 52' 07")
   227.374 (15h 09m 30s) -20.101 (-20d 06' 03")
  ---------------------------------------------
The error box area is 3179 sq. arcmin, and its maximum
dimension is 3.7 deg (the minimum one is 15 arcmin).
The Sun distance was 43 deg.

This box may be improved.

A triangulation map is posted at
http://www.ioffe.ru/LEA/GRBs/GRB151225_T68946/IPN/

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

Y. Kawakubo, A. Yoshida, T. Sakamoto, I. Takahashi, K. Senuma, 
M. Moriyama, 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:

A possible short-duration GRB 151225A (trigger #1135105601) triggered the CALET 
Gamma-Ray Burst Monitor (CGBM) at 19:09:10.34 UT on 25 December 2015.  The clear 
burst signal was detected by all three CGBM instruments.  

The light curve of the SGM shows a double peak structure.  The initial spike 
starts and peaks around T0.  The second peak starts around T0+0.5 sec, peaks 
around T0+0.7 sec and ending around T0+1.5 sec.  Above 500 keV band, the first 
peak looks insignificant, instead the 2nd peak is clearly visible.  Based on the 
light curve inspection, the signal is clearly visible up to ~1.5 MeV.  Low energy 
data show no significant soft extended emission above ~7 keV.  The T90 duration 
measured by the SGM data is 1.87 +- 0.13 sec (40-450 keV).  

Identification of this possible short event by other spacecraft is strongly encouraged.