GRB 151228B

E. Mazaeva (IKI), R. Inasaridze (AbAO), V. Ayvazian (AbAO), A. Volnova 
(IKI), I. Molotov (KIAM), A. Pozanenko (IKI) report on behalf of larger 
GRB follow-up collaboration:

We observed the field of the Swift  GRB 151228B (Lien et  al., GCN 
18739
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 ) with AS-32 (0.7m) telescope of Abastumani Observatory starting on 
Dec. 29 (UT) 14:38:28.  We do not detect any source within
enhanced XRT error circle (Goad et al., GCN 18743
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 ):

Date       UT start     t-T0          Filter Exp.    UL(3 sigma)
            (mid, days)                       (s)

2015-12-29 14:38:28     0.69297       None  35*120   22.1

The photometry is based on nearby SDSS-DR9 stars:
SDSS-DR9_id             R_Lupton
J225738.02+080336.8 18.18
J225747.18+080340.7 16.86
J225736.71+080628.4 17.72
J225752.89+080438.1 17.46
J225726.79+080540.6 16.99

A.A. Breeveld (UCL-MSSL), S.W.K. Emery (UCL-MSSL) and A.Y. Lien 
(GSFC/UMBC) report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 151228B
49 s after the BAT trigger (Lien et al., GCN Circ. 18739
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 ).
A fading source consistent with the XRT position (Goad et al. GCN Circ. 
18743
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 ) is detected in the initial UVOT exposures in the white filter.

The preliminary UVOT position is:
     RA  (J2000) =  22:57:41.87 = 344.42446 (deg.)
     Dec (J2000) = +08:04:53.7  =   8.08159 (deg.)
with an estimated uncertainty of 0.58 arc sec. (radius, 90% confidence).

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               70          220          147         19.62 �� 0.19
v                   49         6132          306        >19.2
b                  537         5514          274        >20.0
u                  282         5309          520        >20.1
w1                 661         6373          303        >20.1
m2                6137         6337          197        >19.9
w2                1386         5927          216        >20.8

The magnitudes in the table are not corrected for the Galactic 
extinction due to the reddening of E(B-V) = 0.06 in the direction of the 
burst (Schlegel et al. 1998).

Y.Muraki, T.Fujiwara, T. Yoshii, Y. Saito, Y. Tachibana,
Y.Yano, S.Harita, Y. Yatsu, and N. Kawai (Tokyo Tech)

report on behalf of the MITSuME collaboration:

We searched for the optical counterpart of GRB 151228B (A. Y. Lien et al., GCN Circular #18739
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 ) with the 
optical three color (g', Rc, and Ic) CCD cameras attached to the MITSuME 50 cm
telescope of Akeno Observatory, Yamanashi, Japan.

The observation started on 2015-12-29 09:55:19 UT (~11.1 h after the burst).
We did not find any new point source within Swift/XRT circle in all three bands.

We obtained following limits for the magnitudes.

T0+[sec] MID-UT T-EXP[sec] g' Rc Ic
-------------------------------------------------------------
40085  10:37:06  2507  > 20.9  > 20.6  > 19.6
-------------------------------------------------------------
T0+ : Elapsed time after the burst 
T-EXP: Total Exposure time 
We used GSC2.3 catalog for flux calibration.

P Veres (UAH) reports on behalf of the Fermi GBM Team:

"At 22:47:12.59 UT on 28 December 2015, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 151228B (trigger 473035636 / 151228949).
which was also detected by the Swift/BAT (Lien et al. 2015, GCN 18739
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 ).
The GBM on-ground location is consistent with the Swift position.

The angle from the Fermi LAT boresight at the GBM trigger time is 107 degrees.

The GBM light curve shows of a single pulse
with a duration (T90) of about 29 s (50-300 keV). There was a
Solar flare coincident with this GRB which affected the low
energy channels of GBM. For this reason the spectral analysis
was carried out for photon energies above 40 keV.
The time-averaged spectrum from T0-6.1 s to T0+21.5 s is
adequately fit by a power law function with an exponential
high-energy cutoff.  The power law index is -1.1 +/- 0.4 and
the cutoff energy, parameterized as Epeak, is 93 +/- 9 keV.

The event fluence (10-1000 keV) in this time interval is
(3.6 +/- 0.4)E-6 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+2.3 s in the 10-1000 keV band
is 7.6 +/- 0.9 ph/s/cm^2.

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

[GCN OPS NOTE(29dec15): Per author's request, the "29A" in the Subject-line
was changed to "28B", and the "2008"  was changed to "2015" in the citation.]

H. A. Krimm (GSFC/USRA), S. D. Barthelmy (GSFC), J. R. Cummings (GSFC/UMBC),
N. Gehrels (GSFC), A. Y. Lien (GSFC/UMBC), C. B. Markwardt (GSFC),
D. M. Palmer (LANL), T. Sakamoto (AGU), M. Stamatikos (OSU), T. N. Ukwatta (LANL)
(i.e. the Swift-BAT team):
 
Using the data set from T-61 to T+243 sec from the recent telemetry downlinks,
we report further analysis of BAT GRB 151228B (trigger #668641)
(Lien, et al., GCN Circ. 18739
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 ).  The BAT ground-calculated position is
RA, Dec = 344.431, 8.081 deg, which is 
   RA(J2000)  =  22h 57m 43.4s 
   Dec(J2000) = +08d 04' 52.4" 
with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 100%.
 
The mask-weighted light curve shows a FreD-like peak starting at ~T-10 sec,
peaking at ~T+3 sec, and ending at ~T+65 sec.  T90 (15-350 keV) is
48.0 +- 16.0 sec (estimated error including systematics).
 
The time-averaged spectrum from T-12.56 to T+67.44 sec is best fit by a power law
with an exponential cutoff.  This fit gives a photon index 1.09 +- 0.34, 
and Epeak of 78.1 +- 27.0 keV (chi squared 52.3 for 56 d.o.f.).  For this
model the total fluence in the 15-150 keV band is 2.4 +- 0.1 x 10^-6 erg/cm2
and the 1-sec peak flux measured from T+10.94 sec in the 15-150 keV band is
1.0 +- 0.2 ph/cm2/sec.  A fit to a simple power law gives a photon index
of 1.66 +- 0.08 (chi squared 61.6 for 57 d.o.f.).  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/668641/BA/

J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), A. D'Ai
(INAF-IASFPA), A. Maselli  (INAF-IASFPA), A. Melandri (INAF-OAB), J.A.
Kennea (PSU), B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU), C.
Pagani (U. Leicester) and A.Y. Lien report on behalf of the Swift-XRT
team:

We have analysed 10 ks of XRT data for GRB 151228B (Lien et al. GCN
Circ. 18739
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 ), from 40 s to 29.0 ks after the  BAT trigger. The data
comprise 194 s in Windowed Timing (WT) mode (the first 8 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. 18743
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 ).

The late-time light curve (from T0+4.7 ks) can be modelled with a
power-law decay with a decay index of alpha=0.68 (+0.22, -0.21).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.43 (+/-0.08). The
best-fitting absorption column is  1.78 (+0.21, -0.20) x 10^21 cm^-2,
in excess of the Galactic value of 5.7 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.94 (+0.14, -0.12)
and a best-fitting absorption column consistent with the Galactic
value. The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 3.4 x 10^-11 (3.7 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     5.7 (+2.3, -0.0) x 10^20 cm^-2
Galactic foreground: 5.7 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.94 (+0.14, -0.12)

If the light curve continues to decay with a power-law decay index of
0.68, the count rate at T+24 hours will be 8.8 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.0 x
10^-13 (3.2 x 10^-13) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00668641.

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

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

Using 2601 s of XRT Photon Counting mode data and 4 UVOT
images for GRB 151228B, 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 = 344.42460, +8.08148 which is equivalent
to:

RA (J2000): 22h 57m 41.90s
Dec (J2000): +08d 04' 53.3"

with an uncertainty of 1.8 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.

A. Y. Lien (GSFC/UMBC), S. D. Barthelmy (GSFC), D. N. Burrows (PSU),
F. E. Marshall (NASA/GSFC), L. M. McCauley (PSU) and
K. L. Page (U Leicester) report on behalf of the Swift Team:

At 22:47:14 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 151228B (trigger=668641).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 344.441, +8.076, which is 
   RA(J2000)  =  22h 57m 46s
   Dec(J2000) = +08d 04' 34"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows FRED-like peak
structure with a duration of about 40 sec.  The peak count rate
was ~2500 counts/sec (15-350 keV), at ~1 sec after the trigger. 

The XRT began observing the field at 22:48:04.4 UT, 49.6 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source with an enhanced position: RA, Dec
344.4247, 8.0816 which is equivalent to:
   RA(J2000)  = 22h 57m 41.93s
   Dec(J2000) = +08d 04' 53.7"
with an uncertainty of 2.7 arcseconds (radius, 90% containment). This
location is 61 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 http://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 5.74
x 10^20 cm^-2 (Willingale et al. 2013). 

The initial flux in the 2.5 s image was 6.74e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 57 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =  22:57:41.85 = 344.42437
  DEC(J2000) = +08:04:54.0  =   8.08166
with a 90%-confidence error radius of about 0.80 arc sec. This position is 3.8
arc sec. from the center of the XRT error circle. The estimated magnitude is
19.74 with a 1-sigma error of about  0.17. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.06. 

Burst Advocate for this burst is A. Y. Lien (amy.y.lien 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/too.html.)