GRB 151210A

P. A. Evans (U Leicester), S. D. Barthelmy (GSFC), J. A. Kennea (PSU),
D. M. Palmer (LANL), B. Sbarufatti (INAF-OAB/PSU) and
M. H. Siegel (PSU) report on behalf of the Swift Team:

At 03:12:56 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 151210A (trigger=666931).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 65.182, -71.230 which is 
   RA(J2000)  =  04h 20m 44s
   Dec(J2000) = -71d 13' 48"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a couple of peaks
with a duration of about 60 sec.  The peak count rate
was ~1000 counts/sec (15-350 keV), at ~14 sec after the trigger. 

The XRT began observing the field at 03:15:26.7 UT, 150.2 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source with an enhanced position: RA, Dec 65.1177,
-71.2509 which is equivalent to:
   RA(J2000)  = 04h 20m 28.25s
   Dec(J2000) = -71d 15' 03.4"
with an uncertainty of 2.1 arcseconds (radius, 90% containment). This
location is 105 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 1.30
x 10^21 cm^-2 (Willingale et al. 2013). 

UVOT took a finding chart exposure of 150 seconds with the White
filter  starting 150 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 none of  the XRT error circle. 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 0.12. 

Burst Advocate for this burst is P. A. Evans (pae9 AT star.le.ac.uk). 
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.)

Yanhui Han (NAOC/CAS, HUST), Thomas Kruehler (ESO), Dong Xu (NAOC/CAS), 
Youdong Hu (IAA-CSIC), and Yin Qing (Geneva Observatory) report:

We observed the field of GRB 151210A (Evans et al., GCN 18678
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 ) with the 
1-m LCOGT telescope at Cerro Tololo, Chile. Observations started 
03:34:06 UT on 2015-12-10 (i.e., 21.2 mins after the GRB), and we 
obtained 10x240s R-band images.

Within the XRT error circle (Evans et al., GCN 18678
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 ), an uncatalogued 
optical source is detected in our stacked image at coordinates:

R. A. = 04:20:28.048
Dec. = -71:15:02.53

with an error radius of ~0.5 arcsec. The object roughly has R~21.5 mag, 
calibrated with the nearby USNO B1 field. We thus consider that the 
source is very likely the optical afterglow of GRB 151220A.

P. Wiseman, T. Kruehler (both MPE Garching), D. A. Kann (TLS
Tautenburg), C. Delvaux, and J. Greiner (both MPE Garching) report on
behalf of the GROND team:

We observed the field of GRB 151210A (Swift trigger 666931; Evans et
al., GCN #18678
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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 03:17:35 UT on 10/12/2015, 279 sec after the
GRB trigger, and are ongoing.

We detect a source not seen in the DSS at the edge of the refined XRT
error circle (Evans et al., GCN #18678
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 ) at:

RA (J2000) = 04:20:27.91
Dec. (J2000) = -71:15:02.6

with an error of 0".5 in each coordinate. We suggest that this is the
afterglow of the GRB, as we also detect fading between observations. This
source is also seen by LCOGT (Han et al., GCN #18679
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 ).

Based on observations with an integration time of 594 sec (griz) and
540 sec (JHK) centered 54 min after the GRB trigger, we derive
magnitudes

g' = 23.9 +/- 0.2  mag,
r' = 22.1 +/- 0.1 mag,
i' = 21.7 +/- 0.1 mag,
z' = 21.2 +/- 0.1 mag,
J = 20.3 +/- 0.2 mag, and
H = 19.8 +/- 0.2 mag.

The given limits are derived based on calibrating the images against
GROND zero points and 2MASS field stars and are not corrected for the
Galactic foreground extinction corresponding to a reddening of E_(B-
V)= 0.11 in the direction of the burst (Schlafly & Finkbeiner 2011).

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

Using 1491 s of XRT Photon Counting mode data and 2 UVOT
images for GRB 151210A, 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 = 65.11644, -71.25075 which is equivalent
to:

RA (J2000): 04h 20m 27.95s
Dec (J2000): -71d 15' 02.7"

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.

D.N. Burrows (PSU), T.G.R. Roegiers (PSU), A.P. Beardmore (U.
Leicester), S.L. Gibson (U. Leicester), A. Melandri (INAF-OAB), P.
D'Avanzo (INAF-OAB), V. D'Elia (ASDC), L.M. McCauley (PSU) and P.A.
Evans report on behalf of the Swift-XRT team:

We have analysed 7.1 ks of XRT data for GRB 151210A (Evans et al. GCN
Circ. 18678
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 ), from 137 s to 23.7 ks after the  BAT trigger. The data
comprise 145 s in Windowed Timing (WT) mode (the first 6 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 Beardmore
et al. (GCN Circ. 18681
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 ).

The light curve can be modelled with an initial power-law decay with an
index of alpha=5.1 (+0.9, -0.8), followed by a break at T+280 s to an
alpha of 0.56 (+/-0.11).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.89 (+0.23, -0.09). The
best-fitting absorption column is  consistent with the Galactic value
of 1.3 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 3.5 x 10^-11 (4.4 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.3 (+/-0.5) x 10^21 cm^-2
Galactic foreground: 1.3 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.89 (+0.23, -0.09)

If the light curve continues to decay with a power-law decay index of
0.56, the count rate at T+24 hours will be 9.1 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.2 x
10^-13 (4.0 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/00666931.

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

S. D. Barthelmy (GSFC), J. R. Cummings (GSFC/UMBC), P. A. Evans (U Leicester),
N. Gehrels (GSFC), H. A. Krimm (GSFC/USRA), 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-239 to T+963 sec from recent telemetry downlinks,
we report further analysis of BAT GRB 151210A (trigger #666931)
(Evans, et al., GCN Circ. 18678
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 ).  The BAT ground-calculated position is
RA, Dec = 65.080, -71.225 deg, which is 
   RA(J2000)  =  04h 20m 19.1s 
   Dec(J2000) = -71d 13' 28.9" 
with an uncertainty of 1.8 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 60%.
 
The mask-weighted light curve shows several peaks starting at ~T-55 sec,
with the brightest peaks at ~T+14 and ~T+38 sec, and ending at ~T+60sec.
T90 (15-350 keV) is 94.9 +- 10.4 sec (estimated error including systematics).
 
The time-averaged spectrum from T-57.19 to T+48.36 sec is best fit by a power law
with an exponential cutoff.  This fit gives a photon index 0.82 +- 0.79, 
and Epeak of 49.4 +- 15.3 keV (chi squared 53.7 for 56 d.o.f.).  For this
model the total fluence in the 15-150 keV band is 1.3 +- 0.2 x 10^-6 erg/cm2
and the 1-sec peak flux measured from T+38.82 sec in the 15-150 keV band is
0.8 +- 0.2 ph/cm2/sec.  A fit to a simple power law gives a photon index
of 1.83 +- 0.15 (chi squared 59.89 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/666931/BA/

S. R. Oates (IAA-CSIC/UCL-MSSL) and P. A. Evans (U Leicester)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 151210A
153 s after the BAT trigger (Evans et al., GCN Circ. 18678
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 ).
No optical afterglow consistent with the optical position
(Han et al., GCN Circ. 18679
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 ; Wiseman et al. GCN Circ. 18680
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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           153          302          147         >21.1
u_FC               311          560          246         >20.5
white              153         7227          805         >22.1
v                  640         7549          442         >20.3
b                  566         7023          510         >20.9
u                  311         6817          736         >20.8
w1                 690         6613          313         >20.4
m2                 840         6407          235         >19.8
w2                 616         7433          490         >20.7

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.12 in the direction of the burst
(Schlegel et al. 1998).