GRB 171216A

F. E. Marshall (NASA/GSFC) reports on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 171216A
124 s after the BAT trigger (Marshall et al., GCN Circ. 22256
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 ).
No optical afterglow consistent with the XRT position
(Goad et al., GCN Circ. 22257
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 )
is detected in the initial UVOT exposures.
Preliminary 3-sigma upper limits using the UVOT photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first
finding chart (FC) exposure and subsequent exposures are:

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

white_FC           124          274          147         >21.3
u_FC               282          532          246         >18.4
v                  613        17562          371         >17.8
b                  539         6997          359         >19.8
u                  282         6886          678         >18.6
w1                 662         6681          452         >19.3
m2                 637         6476          274         >19.3
w2                 588        12734          720         >20.1

These preliminary results may be affected by the presence of
a bright source about 10" from the XRT position.

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.26 in the direction of the burst
(Schlegel et al. 1998).

P.A. Evans (U. Leicester), P. D'Avanzo (INAF-OAB), V. D'Elia (ASDC), A.
D'Ai (INAF-IASFPA), J.A. Kennea (PSU), B. Sbarufatti (INAF-OAB/PSU),
D.N. Burrows (PSU), S.L. Gibson (U. Leicester), J.P. Osborne (U.
Leicester) and F.E. Marshall report on behalf of the Swift-XRT team:

We have analysed 8.0 ks of XRT data for GRB 171216A (Marshall et al.
GCN Circ. 22256
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 ), from 134 s to 64.4 ks after the  BAT trigger. The
data are entirely in Photon Counting (PC) mode. The enhanced XRT
position for this burst was given by Goad et al. (GCN Circ. 22257
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 ).

The light curve can be modelled with an initial power-law decay with an
index of alpha=2.9 (+0.6, -0.3), followed by a break at T+456 s to an
alpha of 0.86 (+0.10, -0.09).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.76 (+0.16, -0.09). The
best-fitting absorption column is  consistent with the Galactic value
of 2.8 x 10^21 cm^-2 (Willingale et al. 2013). The counts to observed
(unabsorbed) 0.3-10 keV flux conversion factor deduced from this
spectrum  is 4.2 x 10^-11 (5.3 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.8 (+/-0.3) x 10^21 cm^-2
Galactic foreground: 2.8 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.76 (+0.16, -0.09)

If the light curve continues to decay with a power-law decay index of
0.86, the count rate at T+24 hours will be 4.0 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.7 x
10^-13 (2.1 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/00797441.

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

C. B. Markwardt (GSFC), S. D. Barthelmy (GSFC),
J. R. Cummings (CPI), H. A. Krimm (NSF/USRA),
A. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/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-240 to T+302 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 171216A (trigger #797441)
(Marshall et al., GCN Circ. 22256
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 ).  The BAT ground-calculated position is
RA, Dec = 212.013, -50.500 deg which is
  RA(J2000)  =  14h 08m 03.0s
  Dec(J2000) = -50d 30' 01.2"
with an uncertainty of 2.1 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 63%.

The mask-weighted light curve shows a weak single-peaked structure that
starts at ~T0, peaks at ~T+3 s, and ends at ~ T+30 s. T90 (15-350 keV)
is 30.0 +- 7.5 sec (estimated error including systematics).

The time-averaged spectrum from T-0.45 to T+33.47 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.63 +- 0.30.  The fluence in the 15-150 keV band is 4.8 +- 1.0 x 10^-7 erg/cm2.
The 1-sec peak photon flux measured from T+3.05 sec in the 15-150 keV band
is 0.8 +- 0.2 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/797441/BA/

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

Using 3440 s of XRT Photon Counting mode data and 7 UVOT
images for GRB 171216A, 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 = 211.99069, -50.48474 which is equivalent
to:

RA (J2000): 14h 07m 57.77s
Dec (J2000): -50d 29' 05.1"

with an uncertainty of 1.5 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.

F. E. Marshall (NASA/GSFC), J. R. Cummings (NASA/UMBC),
A. Deich (PSU), J.D. Gropp (PSU), J. A. Kennea (PSU),
A. Y. Lien (GSFC/UMBC), K. L. Page (U Leicester) and
D. M. Palmer (LANL) report on behalf of the Swift Team:

At 03:06:59 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 171216A (trigger=797441).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 212.078, -50.473 which is 
   RA(J2000) = 14h 08m 19s
   Dec(J2000) = -50d 28' 22"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex
structure with a duration of ~20 sec.  The peak count rate
was ~600 counts/sec (15-350 keV), at ~4 sec after the trigger. 

The XRT began observing the field at 03:08:54.7 UT, 115.2 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source with an enhanced position: RA, Dec
211.9924, -50.4851 which is equivalent to:
   RA(J2000)  = 14h 07m 58.17s
   Dec(J2000) = -50d 29' 06.3"
with an uncertainty of 2.1 arcseconds (radius, 90% containment). This
location is 200 arcseconds from the BAT onboard position. 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 2.84
x 10^21 cm^-2 (Willingale et al. 2013). 

The initial flux in the 2.5 s image was 5.37e-10 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 123 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 100% of
the XRT error circle. Because of the density of catalogued stars, further
analysis is required to report an upper limit for any afterglow in the
sub-image. The 8'x8' region for the list of sources generated on-board covers
100% of the XRT error circle. Because of the density of catalogued stars,
further analysis is required to report an upper limit for any afterglow in the
region. No correction has been made for the expected extinction corresponding
to E(B-V) of 0.26. 

Burst Advocate for this burst is F. E. Marshall (marshall AT milkyway.gsfc.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.)