GRB 100316C

M. Stamatikos (OSU/NASA/GSFC), W. H. Baumgartner (GSFC/UMBC),
A. P. Beardmore (U Leicester), S. Campana (INAF-OAB),
P. A. Evans (U Leicester), J. M. Gelbord (PSU),
C. Guidorzi (U Ferrara), S. T. Holland (CRESST/USRA/GSFC),
O. M. Littlejohns (U Leicester), R. Margutti (Univ Bicocca&OAB),
C. B. Markwardt (CRESST/GSFC/UMD), P. T. O'Brien (U Leicester),
C. Pagani (U Leicester), K. L. Page (U Leicester),
D. M. Palmer (LANL), P. Romano (INAF-IASFPA), M. H. Siegel (PSU),
R. L. C. Starling (U Leicester) and L. Vetere (PSU) report on behalf
of the Swift Team:

At 08:57:59 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 100316C trigger=416115).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 32.291, -68.009 which is 
   RA(J2000) = 02h 09m 10s
   Dec(J2000) = -68d 00' 31"
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 7 sec.  The peak count rate
was ~1500 counts/sec (15-350 keV), at ~1 sec after the trigger. 

The XRT began observing the field at 08:59:17.9 UT, 78.3 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source with an enhanced position: RA, Dec 32.2869, -67.9914 which
is equivalent to:
   RA(J2000)  = 02h 09m 8.85s
   Dec(J2000) = -67d 59' 29.1"
with an uncertainty of 2.4 arcseconds (radius, 90% containment). This
location is 63 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 in excess of the Galactic value (3.94e+20
cm-2, Kalberla et al. 2005), with an excess column of 2.6
(+2.05/-1.79) x 1021 cm-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 81 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 overlap of the 8'x8' region for the list of sources generated on-board and
the BAT error circle is uncertain. No correction has been made for the expected
extinction corresponding to E(B-V) of 0.04. 

Burst Advocate for this burst is M. Stamatikos (Michael.Stamatikos-1 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. Jelinek (IAA-CSIC), A. de Ugarte Postigo (INAF-OAB),
A.J. Castro-Tirado (IAA-CSIC), J. Gorosabel (IAA-CSIC),
P. Kubanek (IPL UV, IAA-CSIC), R. Cunniffe (IAA-CSIC),
S. Guziy (IAA-CSIC), P. Yock  (Auckland Univ.),
W.H. Allen (Vintage Lane Obs.), I. Bond (Massey Univ.),
G. Christie (Stardome Obs.),
on behalf of a larger collaboration report:

"We have observed the field of GRB 100316C (Stamatikos et al.,
GCN 10491
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 ) using the 0.6m Yock-Allen robotic telescope
(BOOTES-3) located in Blenheim, New  Zealand.  Observations
began at  8:58:47 UT (49s from GRB onset and 4 s from the GCN
alert receipt).  We do not detect any  source within the XRT
error circle in a 20 x 2 second combined unfiltered image down
to a 3 sigma limiting magnitude of 18.5."

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

Using 1879 s of XRT Photon Counting mode data and 2 UVOT
images for GRB 100316C, 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 = 32.28660, -67.99098 which is equivalent
to:

RA (J2000): 02h 09m 8.78s
Dec (J2000): -67d 59' 27.5"

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.

K.L. Page (U. Leicester) and M. Stamatikos (OSU/NASA/GSFC) report on 
behalf of the Swift-XRT team:

We have analysed 5.7 ks of XRT data for GRB 100316C (Stamatikos et al. GCN 
Circ. 10491
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 ), from 87 s to 13.5 ks after the BAT trigger. The data are 
entirely in Photon Counting (PC) mode. The enhanced XRT position for this 
burst was given by Evans et al. (GCN Circ. 10506
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 ).

The light curve can be modelled with a doubly-broken power-law, with 
alpha1 = 0.28 (+0.66,-1.18) until T = 258 (+87/-66) s after the trigger, 
at which point the decay steepens to alpha2 = 2.53 (+1.18, -0.44). At T = 
883 (+1857, -271) s, the decay flattens to alpha3 = 0.76 (+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 1.6 (+0.5, -0.4). The best-fitting 
absorption column is 1.8 (+1.6, -1.3) x 10^21 cm^-2, in excess of the Galactic 
value of 3.9 x 10^20 cm^-2 (Kalberla et al. 2005). The counts to observed 
(unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 
5.1 x 10^-11 (6.2 x 10^-11) erg cm^-2 count^-1.

If the light curve continues to decay with a power-law decay index of 0.76, the 
count rate at T+24 hours will be 0.0013 count s^-1, corresponding to an 
observed (unabsorbed) 0.3-10 keV flux of 6.6 x 10^-14 (8.1 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/00416115.

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

D. M. Palmer (LANL), S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC),
J. R. Cummings (GSFC/UMBC), N. Gehrels (GSFC), H. A. Krimm (GSFC/USRA),
C. B. Markwardt (GSFC/UMD), T. Sakamoto (GSFC/UMBC), M. Stamatikos (GSFC/ORAU),
J. Tueller (GSFC), T. N. Ukwatta (GWU)
(i.e. the Swift-BAT team):
 
Using the data set from T-60 to T+243 sec from recent telemetry downlinks,
we report further analysis of BAT GRB 100316C (trigger #416115)
(Stamatikos, et al., GCN Circ. 10491
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 ).  The BAT ground-calculated position is
RA, Dec = 32.309, -67.992 deg which is 
   RA(J2000)  =  02h 09m 14.2s 
   Dec(J2000) = -67d 59' 30.3" 
with an uncertainty of 2.0 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 100%.
 
The mask-weighted light curve shows a roughly symmetric peak starting
at ~T-3 sec, peaking at ~T+3 sec, and ending at ~T+12 sec.  T90 (15-350 keV)
is 9.3 +- 1.7 sec (estimated error including systematics).
 
The time-averaged spectrum from T-3.0 to T+7.0 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.58 +- 0.26.  The fluence in the 15-150 keV band is 1.8 +- 0.3 x 10^-7 erg/cm2.
The 1-sec peak photon flux measured from T+2.12 sec in the 15-150 keV band
is 0.4 +- 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/416115/BA/

S. R. Oates (MSSL-UCL) and M. Stamatikos (OSU/NASA/GSFC)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of
GRB 100316C 81s after the BAT trigger (Stamatikos et al., GCN Circ. 10491
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 ).
We do not detect any source at the enhanced Swift XRT position
(Evans et al. GCN Circ. 10506
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 ).

The 3-sigma upper limits for the finding chart exposures (FC)
and summed images are:

Filter   T_start   T_stop   Exp(s)  Mag (3-sigma upper limit)
-------------------------------------------------------------
white (FC)  81    231        147       > 20.52
white       521   7450       677       > 21.63
u     (FC)  239   489        246       > 19.87
u           644   7039       529       > 20.22
v           570   12991      1326      > 20.45
b           495   7245       549       > 20.72
uvw1        620   6834       333       > 20.24
uvm2        595   13530      741       > 20.60
uvw2        546   7656       471       > 20.57
-------------------------------------------------------------

The quoted upper limits have not been corrected for the expected
Galactic extinction along the line of sight of E_(B-V) = 0.04 mag.
All photometry is on the UVOT photometric system described in Poole et
al. (2008, MNRAS, 383, 627).

Paulo Afonso (MPE Garching), Adria Updike (Clemson University), Marco
Nardini, Robert Filgas, Abdullah Yoldas, and Jochen Greiner (MPE Garching)
report on behalf of the GROND team:

We observed the field of GRB 100316C (Swift trigger 416115; Stamatikos et
al. GCN 10491
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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 ESO/MPI telescope at La Silla
Observatory (Chile).

Observations started at 00:45 UT on March 17, 2010, 15.8 hours after the
GRB trigger, and are continuing. They were performed at an average seeing
of 1" and at an average airmass of 2.

We found a DSS uncataloged source at the edge of the 1.8 Swift-XRT error
circle reported by Evans et al. (GCN 10506
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 ) at

RA (J2000.0) = 02 h 09 m 09.11 s

DEC (J2000.0) = -67d 59' 26.8"

with an uncertainty of 0.5" in each coordinate.

Based on the first 31 min of total exposures in g'r'i'z', we estimate
preliminary magnitudes (all in AB system) of

g' = 23.7 +/- 0.1 mag,

r' = 24.4 +/- 0.1 mag,

i' > 23.6 mag,

z' > 23.9 mag.

Further observations are necessary to confirm variability.  If this is
indeed the afterglow of GRB 100316C, the colors indicate a redshift less
than 3.5.  Given magnitudes are calibrated against GROND zeropoints and
are not corrected for the expected Galactic foreground extinction
corresponding to a reddening of E_(B-V)=0.03 mag in the direction of the
burst (Schlegel et al. 1998).  JHK reduction is ongoing.