GRB 161220A

V. D'Elia (ASDC), A. D'Ai (INAF-IASFPA), F. E. Marshall (NASA/GSFC),
K. L. Page (U Leicester), T. Sakamoto (AGU),
B. Sbarufatti (INAF-OAB/PSU) and T. N. Ukwatta (LANL) report on behalf
of the Swift Team:

At 08:33:30 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 161220A (trigger=727610).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 350.817, -47.502 which is 
   RA(J2000) = 23h 23m 16s
   Dec(J2000) = -47d 30' 07"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a FRED-like 
structure with a duration of about 12 sec.  The peak count rate
was ~1285 counts/sec (15-350 keV), at ~1 sec after the trigger. 

The XRT began observing the field at 08:34:39.2 UT, 69.1 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source located at RA, Dec 350.84774,
-47.49773, which is equivalent to:
   RA(J2000)  = 23h 23m 23.46s
   Dec(J2000) = -47d 29' 51.8"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 76 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 1.19
x 10^20 cm^-2 (Willingale et al. 2013). 

The initial flux in the 2.5 s image was 1.94e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 78 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. The typical 3-sigma upper limit has been
about 19.6 mag. Results from the list of sources
generated on-board are not available at this time. No correction has been made
for the expected extinction corresponding to E(B-V) of 0.01. 

Burst Advocate for this burst is V. D'Elia (delia AT asdc.asi.it). 
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.)

F. E. Marshall (NASA/GSFC) and V. D'Elia (ASDC)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 161220A
78 s after the BAT trigger (D'Elia et al., GCN Circ. 20302
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 ).
No optical afterglow consistent with the XRT position
(D'Elia et al. GCN Circ. 20302
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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            78          228          147         >20.5
white               78         1012          295         >21.0

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.01 in the direction of the burst
(Schlegel et al. 1998).

P. Wiseman, T. Kruehler, and J. Greiner (all MPE Garching) report on
behalf of the GROND team:

We observed the field of GRB 161220A (Swift trigger 727610; D'Elia et al.,
GCN #20302
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 ) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008,
PASP 120, 405) mounted at the 2.2m MPG telescope at the ESO La Silla
Observatory (Chile).

Observations started at 00:36 UT on 2016-12-21, 16 hrs after the GRB
trigger. They were performed at an average seeing of 0".9, and at an
average airmass of 1.2.

Similar to Marshall & D'Elia (GCN #20307
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 ), we detect no credible
optical/NIR afterglow consistent with the XRT position. Based on combined
images with 14.5 min of total exposures in g'r'i'z'
and 12.0 min in JHK at a mid-time of 00:45 UT on 2016-12-21,
we derive the following upper limits (all in AB system).

g' > 22.7 mag
r' > 22.7 mag
i' > 21.3 mag
z' > 21.8 mag
J > 20.8 mag
H > 20.4 mag
K > 17.2 mag

Given magnitudes are calibrated against USNO as well as 2MASS field stars
and are not corrected for the expected Galactic foreground extinction
corresponding to a reddening of E_(B-V) = 0.005 mag in the direction of
the burst (Schlafly & Finkbeiner 2011).

C. Guidorzi (U. Ferrara), S. Kobayashi, I.A. Steele (LJMU), A. Gomboc 
(U. Nova Gorica), C.G. Mundell (U. Bath) on behalf of a large 
collaboration report:

We observed Swift GRB 161220A (D'Elia et al. GCN 20302
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 ) on December 20, 
from 11:02 to 11:13 UT (2.5-2.7 hours since the GRB) with the 2-m LCO 
Faulkes Telescope South in Siding Springs with the SDSS i filter under 
poor sky transparency conditions. Within the XRT error circle we do not 
find any object brighter than i~20 mag (total exposure of 5x120s) as 
calibrated against nearby USNOB1 stars.

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

We have analysed 3.4 ks of XRT data for GRB 161220A (D'Elia et al. GCN
Circ. 20302
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 ), from 60 s to 58.3 ks after the  BAT trigger. The data
comprise 85 s in Windowed Timing (WT) mode (the first 7 s were taken
while Swift was slewing) with the remainder in Photon Counting (PC)
mode. The refined XRT position is RA, Dec = 350.8479, -47.4978 which is
equivalent to:

RA (J2000): 23 23 23.49
Dec(J2000): -47 29 52.2

with an uncertainty of 3.6 arcsec (radius, 90% confidence).

The light curve can be modelled with a series of power-law decays. The
light curve initially rises, with an index alpha=-1.5 (+0.0, -2.0). At
T+71.0 s it breaks to an alpha of 4.75 (+0.51, -0.29) before breaking
again at T+217 s to a final decay with index alpha=0.82 (+0.13, -0.11).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 3.57 (+0.20, -0.19). The
best-fitting absorption column is  3.2 (+/-0.4) x 10^21 cm^-2, in
excess of the Galactic value of 1.2 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 2.1 x 10^-11 (9.9 x 10^-11) erg
cm^-2 count^-1. 

A summary of the WT-mode spectrum is thus:
Total column:	     3.2 (+/-0.4) x 10^21 cm^-2
Galactic foreground: 1.2 x 10^20 cm^-2
Excess significance: 12.4 sigma
Photon index:	     3.57 (+0.20, -0.19)

If the light curve continues to decay with a power-law decay index of
0.82, the count rate at T+24 hours will be 2.7 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.7 x
10^-14 (2.6 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/00727610.

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

T. Sakamoto (AGU), S. D. Barthelmy (GSFC),
J. R. Cummings (CPI), V. D'Elia (ASDC),
N. Gehrels (GSFC), H. A. Krimm (NSF/USRA),
A. Y. Lien (GSFC/UMBC), C. B. Markwardt (GSFC),
D. M. Palmer (LANL), 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 downlink, we report further analysis
of BAT GRB 161220A (trigger #727610) (D'Elia, et al.,
GCN Circ. 20302
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 ).  The BAT ground-calculated position
is RA, Dec = 350.841, -47.480 deg which is
   RA(J2000)  =  23h 23m 21.7s
   Dec(J2000) = -47d 28' 47.3"
with an uncertainty of 2.0 arcmin, (radius, sys+stat,
90% containment). The partial coding was 100%.

The mask-weighted light curve shows a single episode
that starts around T-4 sec, peaks around T+3 sec and
ends at ~T+4 sec. T90 (15-350 keV) is 6 +- 1 sec
(estimated error including systematics).

The time-averaged spectrum from T-2.24 to T+4.66 sec
is best fit by a simple power-law model.  The power
law index of the time-averaged spectrum is 2.01 +- 0.25.
The fluence in the 15-150 keV band is
1.7 +- 0.3 x 10^-07 erg/cm2. The 1-sec peak photon flux
measured from T+2.42 sec in the 15-150 keV band
is 0.5 +- 0.1 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/727610/BA/