GRB 181203A

E. Troja (NASA/GSFC/UMCP), A. P. Beardmore (U Leicester),
A. A. Breeveld (UCL-MSSL), J.D. Gropp (PSU), J. A. Kennea (PSU),
A. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/GSFC),
M. J. Moss (George Washington University), K. L. Page (U Leicester),
D. M. Palmer (LANL) and A. Tohuvavohu (PSU) report on behalf of the
Neil Gehrels Swift Observatory Team:

At 12:08:47 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 181203A (trigger=874475).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 108.369, -39.769 which is 
   RA(J2000) = 07h 13m 29s
   Dec(J2000) = -39d 46' 07"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single-peaked
structure with a duration of about 20 sec.  The peak count rate
was ~1000 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 12:10:01.1 UT, 73.3 seconds after
the BAT trigger. The position determined from promptly downlinked data
differs significantly from the on-board position, suggesting that the
XRT may have centroided on a cosmic ray; the initial XRT position
notice should be treated with caution. Using promptly downlinked data
we find a fading, uncatalogued X-ray source with an enhanced position:
RA, Dec 108.3491, -39.8016 which is equivalent to:
   RA(J2000)  = 07h 13m 23.78s
   Dec(J2000) = -39d 48' 05.6"
with an uncertainty of 2.4 arcseconds (radius, 90% containment). This
location is 129 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 2.26
x 10^21 cm^-2 (Willingale et al. 2013). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 79 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. 
The 8'x8' region for the list of sources generated on-board covers 100% of the
XRT error circle. The list of sources is typically complete to about 18 mag. No
correction has been made for the expected extinction corresponding to E(B-V) of
0.24. 

Burst Advocate for this burst is E. Troja (eleonora.troja 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.)

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

Using 867 s of XRT Photon Counting mode data and 1 UVOT
images for GRB 181203A, 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 = 108.34857, -39.80187 which is equivalent
to:

RA (J2000): 07h 13m 23.66s
Dec (J2000): -39d 48' 06.7"

with an uncertainty of 2.0 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.

S. D. Barthelmy (GSFC), J. R. Cummings (CPI),
H. A. Krimm (NSF/USRA), A. Y. Lien (GSFC/UMBC),
C. B. Markwardt (GSFC), D. M. Palmer (LANL),
T. Sakamoto (AGU), M. Stamatikos (OSU),
E. Troja (NASA/GSFC/UMCP), T. N. Ukwatta (LANL)
(i.e. the Swift-BAT team):

Using the data set from T-240 to T+962 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 181203A (trigger #874475)
(Troja et al., GCN Circ. 23493
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 ).  The BAT ground-calculated position is
RA, Dec = 108.341, -39.783 deg which is
   RA(J2000)  =  07h 13m 21.8s
   Dec(J2000) = -39d 46' 57.1"
with an uncertainty of 1.3 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 83%.

The mask-weighted light curve shows a single-peaked structure that starts at
~T-2 s, peaks at ~T+1 s, and ends at ~T+16 s. T90 (15-350 keV) is 15.30 +- 1.75 sec
(estimated error including systematics).

The time-averaged spectrum from T-1.96 to T+15.77 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.79 +- 0.15.  The fluence in the 15-150 keV band is 7.8 +- 0.7 x 10^-7 erg/cm2.
The 1-sec peak photon flux measured from T+1.02 sec in the 15-150 keV band
is 1.1 +- 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/874475/BA/

A. Melandri (INAF-OAB), P. D'Avanzo (INAF-OAB), V. D'Elia (ASDC), B.
Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU), A. Tohuvavohu (PSU),
P.A. Evans (U. Leicester), J.P. Osborne (U. Leicester), K.L. Page (U.
Leicester) and E. Troja report on behalf of the Swift-XRT team:

We have analysed 6.2 ks of XRT data for GRB 181203A (Troja et al. GCN
Circ. 23493
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 ), from 60 s to 64.1 ks after the  BAT trigger. The data
comprise 9 s in Windowed Timing (WT) mode (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. 23494
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 ).

The light curve can be modelled with a power-law decay with a decay
index of alpha=2.74 (+0.27, -0.26).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.84 (+0.29, -0.17). The
best-fitting absorption column is  consistent with the Galactic value
of 2.3 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 3.9 x 10^-11 (5.1 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.3 (+/-0.8) x 10^21 cm^-2
Galactic foreground: 2.3 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.84 (+0.29, -0.17)

If the light curve continues to decay with a power-law decay index of
2.74, the count rate at T+24 hours will be 3.2 x 10^-8 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.2 x
10^-18 (1.6 x 10^-18) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00874475.

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

F. E. Marshall (NASA/GSFC) and E. Troja (NASA/GSFC/UMCP)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 181203A
79 s after the BAT trigger (Troja et al., GCN Circ. 23493
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 ).
No optical afterglow consistent with the XRT position
(Goad et al. GCN Circ. 23494
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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) exposures and subsequent exposures are:

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

white_FC            79          229          147         >20.7
u_FC               293          543          246         >19.4
white               79         1370          373         >21.8
v                  624         1421           97         >18.6
b                  549         1346           78         >20.0
u                  293         1495          324         >19.7
w1                 673         1470           97         >19.0
m2                 648         1445           97         >18.5
w2                 599         1396           97         >19.7

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.24 in the direction of the burst
(Schlegel et al. 1998).

Y.-D. Hu, E. Fernandez-Garcia, A. Ayala and A. J. Castro-Tirado 
(IAA-CSIC), I. Carrasco and C. Perez del Pulgar (Univ. de Malaga), M. D. 
Caballero-Garcia (ASU-CAS, CZ) and R. Querel (NIWA), on behalf of a 
larger collaboration,
report:

The 60cm BOOTES-3/YA robotic telescope at NIWA Lauder in Otago (New 
Zealand) automatically responded to the Swift trigger of GRB 181203A 
(Troja et al. GCNC 23493) ~0.51 h after the burst.  At the position of 
the Swift/XRT enhanced X-ray position (Goad et al. GCNC 23494), no 
optical afterglow is detected down to 20.4 mag on a 360s co-added 
unfiltered image at 0.62 h (middle exposure) after trigger, in agreement 
with the non-detection by Swift/UVOT (Marshall et al. GCNC 23505).

We thank the staff at NIWA for its excellent support.

P. Schady (Univ. of Bath) and J. Bolmer (MPE, Garching) report:

We observed the field of GRB 181203A (Swift trigger 874475; Troja et al., GCN #23493
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 ) simultaneously in
g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120, 405) mounted at the 2.2 m MPG telescope at ESO
La Silla Observatory (Chile).

Observations started at 02:02 UT on 4th December, 13.4 hours after the GRB trigger. They were performed
at an average seeing of 1.6" and at an average airmass of 1.4. 

We found a faint source within the 2.0 arcsec Swift-XRT error circle reported by Goad et al. (GCN #23494
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 ) at

RA (J2000.0) = 07:13:23.705
DEC (J2000.0) = -39:48:05.29

with an uncertainty of 0.12" in each coordinate. The source is detected in all filters apart from the z' band,
and has a preliminary r'-band AB magnitude of 

r' = 23.5 +/- 0.1 mag

The above magnitude is calibrated against GROND zeropoints and is not corrected for the expected
Galactic foreground extinction corresponding to a reddening of E_(B-V)=0.21 in the direction of the
burst (Schlafly & Finkbeiner 2011).

We thank Angela Hempel for the excellent support from La Silla.