GRB 131205A

T. N. Ukwatta (MSU), V. D'Elia (ASDC), C. B. Markwardt (NASA/GSFC),
A. Melandri (INAF-OAB), K. L. Page (U Leicester) and
M. H. Siegel (PSU) report on behalf of the Swift Team:

At 09:18:38 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 131205A (trigger=580267).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 131.609, -60.174 which is 
   RA(J2000) = 08h 46m 26s
   Dec(J2000) = -60d 10' 25"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a multiple peaked
structure with a duration of about 60 sec.  The peak count rate
was ~1300 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 09:20:07.5 UT, 88.7 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source with an enhanced position: RA, Dec 131.6276, -60.1556
which is equivalent to:
   RA(J2000)  = 08h 46m 30.63s
   Dec(J2000) = -60d 09' 20.0"
with an uncertainty of 2.7 arcseconds (radius, 90% containment). This
location is 74 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.  We
cannot determine whether the source is fading at the present time. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 1.50
x 10^21 cm^-2 (Kalberla et al. 2005). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 93 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. 
Data 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 0.26. 

We note that the spacecraft star tracker was out of lock during a 
portion of this GRB, but we believe the burst is real. The position
may be revised after further processing. 

Burst Advocate for this burst is T. N. Ukwatta (tilan.ukwatta AT gmail.com). 
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.)

A. Klotz (IRAP-CNRS-OMP), D. Macpherson (UWA/ICRAR), D. Coward (UWA),
D. Turpin (IRAP-CNRS-OMP), B. Gendre , M. Boer, K. Siellez, H. Dereli ,
O. Bardho (UNS-CNRS-OCA), A. Williams (PO-UWA), R. Martin (PO-UWA)
report:

We imaged the field of GRB 131205A detected by SWIFT
(trigger 580267) with the Zadko robotic telescope (D=100cm)
located at the observatory - Gingin, Australia.

The observations started 3.74h after the GRB trigger
which occured during Australian day time.
The elevation of the field increased from
19 degrees above horizon and weather conditions
were excellent.

We co-added a series of exposures but
we do not detect any OT with a limiting magnitude of:
t0+3.74h to t0+4.45h : Rlim = 20.2

Magnitudes were estimated with the nearby NOMAD1 stars
and are not corrected for galactic dust extinction.

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

Using 2064 s of XRT Photon Counting mode data and 3 UVOT
images for GRB 131205A, 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 = 131.62784, -60.15601 which is equivalent
to:

RA (J2000): 08h 46m 30.68s
Dec (J2000): -60d 09' 21.6"

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.

C. A. Swenson (PSU) and T. N. Ukwatta (MSU)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 131205A
94 s after the BAT trigger (Ukwatta et al., GCN Circ. 15578
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 ).
No optical afterglow consistent with the enhanced XRT position
(Beardmore et al. GCN Circ. 15580
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 ) is detected in the initial UVOT
exposures.  Because the spacecraft attitude was not stable during
the first finding chart exposure, we do not report on that exposure here.

Preliminary 3-sigma upper limits using the UVOT photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the second
finding chart (FC) exposure and subsequent exposures are:

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

u_FC               306          556          246 >20.8
white              586        12884         1171 >21.7
v                  636         6778          471 >20.5
b                  561        12313         1160 >21.5
u                  306        11400         1386 >22.0
w1                 685         7121          385 >20.4
m2                 660         6983          471 >20.4
w2                 611         6574          471 >20.6

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. D'Avanzo (INAF-OAB), V. D'Elia (ASDC), A. Maselli  (INAF-IASFPA), V.
Mangano (PSU), M.C. Stroh (PSU), D.N. Burrows (PSU), J.P. Osborne (U.
Leicester), K.L. Page (U. Leicester), C. Pagani (U. Leicester) and T.N.
Ukwatta report on behalf of the Swift-XRT team:

We have analysed 10 ks of XRT data for GRB 131205A (Ukwatta  et al. GCN
Circ. 15578
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 ),  from 226 s to 24.4 ks after the	BAT trigger. The data
are entirely in Photon Counting (PC) mode. The enhanced XRT position
for this burst was given by Beardmore et al. (GCN. Circ 15580).

The light curve can be modelled with  a power-law decay with a decay
index of alpha=1.10 (+0.09, -0.08).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.1 (+/-0.4). The
best-fitting absorption column is  3.2 (+1.4, -1.2) x 10^21 cm^-2, in
excess of the Galactic value of 1.5 x 10^21 cm^-2 (Kalberla et al.
2005). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 3.9 x 10^-11 (6.8 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     3.2 (+1.4, -1.2) x 10^21 cm^-2
Galactic foreground: 1.5 x 10^21 cm^-2
Excess significance: 2.3 sigma
Photon index:	     2.1 (+/-0.4)

If the light curve continues to decay with a power-law decay index of
1.10, the count rate at T+24 hours will be 9.9 x 10^-4 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.8 x
10^-14 (6.7 x 10^-14) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00580267.

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

T. Sakamoto (AGU), S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC),
J. R. Cummings (GSFC/UMBC), N. Gehrels (GSFC), H. A. Krimm (GSFC/USRA),
A. Y. Lien (NASA/UMBC), C. B. Markwardt (GSFC), D. M. Palmer (LANL),
M. Stamatikos (OSU), J. Tueller (GSFC), T. N. Ukwatta (MSU)
(i.e. the Swift-BAT team):
 
Using the data set from T-239 to T+771 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 131205A (trigger #580267)
(Ukwatta, et al., GCN Circ. 15578
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 ).  The BAT ground-calculated position is
RA, Dec = 131.573, -60.181 deg which is 
   RA(J2000)  =  08h 46m 17.6s 
   Dec(J2000) = -60d 10' 51.5" 
with an uncertainty of 2.8 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 95%.
 
The mask-weighted light curves two peaks starting at ~T-5, peaking at ~T+1,
and ending at ~T+5 sec, and the second peak from ~T+18 to ~T+22 sec.
T90 (15-350 keV) is 37.5 +- 9.6 sec (estimated error including systematics).
 
The time-averaged spectrum from T-18.50 to T+20.41 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.57 +- 0.38.  The fluence in the 15-150 keV band is 2.9 +- 0.7 x 10^-07 erg/cm2.
The 1-sec peak photon flux measured from T+19.30 sec in the 15-150 keV band
is 0.6 +- 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/580267/BA/

S. Schmidl (TLS Tautenburg ), K. Varela and J. Greiner (both MPE Garching)
report on behalf of the GROND team:

We observed the field of the GRB 131205A (Ukwatta et al., GCN #15578
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 )
simultaneously in g'r'i'z'JHKs with GROND (Greiner et al. 2008, PASP 120,
405) mounted at the 2.2 m MPG telescope at La Silla Observatory (Chile).

Observations started at 05:07 UT on 06 December 2013, 19.5 hours after the
GRB. They were performed at an average seeing of 1.0" and at an average
airmass of 1.3.

Based on six co-added observations with individual exposure times of 1500s
in g'r'i'z' and 1200s in JHK, we do not detect any source inside the
enhanced XRT error circle (Beardmore et al., GCN #15580
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 ) down to
preliminary 3 sigma limiting magnitudes (all in the AB system) :

g' > 25.8 mag,
r' > 25.6 mag,
i' > 24.9 mag,
z' > 24.5 mag,
J > 22.1 mag,
H > 21.5 mag,
K > 20.8 mag.

Magnitudes are calibrated against GROND zeropoints (griz) and 2MASS stars
(JHK), and are not corrected for the Galactic foreground extinction
corresponding to a reddening of E_(B-V)=0.25 mag in the direction of the
burst (Schlegel et al. 1998).

We note the marginal detection of a source, located 1.9" from the center
of the XRT circle, at

RA : 08:46:30.86
DEC : -60:09:20.6,

with an uncertainty of 0.2 arcsec and magnitudes of

r = 24.8 +/- 0.2
i = 24.7 +/- 0.3
z = 24.3 +/- 0.3.

This is consistent with a power law and thus could be the afterglow of
GRB131205A, but no statement can be made about fading at this time.
Follow-up observations are required to determine the nature of this
source.

K. Varela, D.A. Kann and J. Greiner (all MPE Garching)
report on behalf of the GROND team:
  
We observed again the field of the GRB 131205A (Ukwatta et al., GCN #15578
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 )
simultaneously in g'r'i'z'JHKs with GROND (Greiner et al. 2008, PASP 120,
405) mounted at the 2.2 m MPG telescope at the La Silla Observatory (Chile).
  
Observations started at 05:22 UT on 11 December 2013. They were performed
at an average seeing of 0.7 arcsec and at an average airmass of 1.3.
  
Based on four co-added observations with individual exposure times of 2214s
in g'r'i'z' and 1800s in JHKs, we measure for the source at the border
of the X-ray error circle (Beardmore et al., GCN #15580
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 ), which was
proposed as potential counterpart (Schmidl et al. 2013, GCN #15585
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 ),
the following magnitudes:
  
r' = 26.5 +/- 0.4
i' = 25.3 +/- 0.4
  
Compared to our earlier measurements of r' = 24.8+/-0.2, i' = 24.7+/-0.3
this implies a fading of 1.7+/-0.5 in r', and 0.6+/-0.5 in i'. 
We conclude that the fading in r' is significant, and propose this
source as the optical counterpart of GRB 131205A.  The low amplitude
in the i'-band may be caused by the emergence of light from a red
host galaxy. Observations with bigger glasses are required to verify
this interpretation.
  
Note that the above r'i' magnitudes are calibrated against GROND zeropoints 
and are not corrected for the Galactic foreground extinction corresponding
to a reddening of E_(B-V)=0.25 mag in the direction of the burst (Schlegel
et al. 1998).