GRB 100316B

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), V. Mangano (INAF-IASFPA),
C. B. Markwardt (CRESST/GSFC/UMD), P. T. O'Brien (U Leicester),
C. Pagani (U Leicester), K. L. Page (U Leicester),
P. Romano (INAF-IASFPA), A. Rowlinson (U Leicester),
M. H. Siegel (PSU), R. L. C. Starling (U Leicester),
G. Tagliaferri (INAF-OAB) and L. Vetere (PSU) report on behalf of the
Swift Team:

At 08:01:36 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 100316B trigger=416103).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is RA, Dec 163.498, -45.480 which is 
   RA(J2000) = 10h 54m 00s
   Dec(J2000) = -45d 28' 47"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a multi-peaked
structure with a duration of about 5 sec.  The peak count rate
was ~2500 counts/sec (15-350 keV), at ~1 sec after the trigger. 

The XRT began observing the field at 08:02:41.1 UT, 64.1 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source located at RA, Dec 163.48846, -45.47220 which
is equivalent to:
   RA(J2000)  = 10h 53m 57.23s
   Dec(J2000) = -45d 28' 19.9"
with an uncertainty of 4.1 arcseconds (radius, 90% containment). This
location is 36 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
9.92e+20 cm-2 (Kalberla et al. 2005). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 66 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	10:53:57.15 = 163.48812
  DEC(J2000) = -45:28:21.6  = -45.47267
with a 90%-confidence error radius of about 0.62 arc sec. This position is 2.1
arc sec. from the center of the XRT error circle. The estimated magnitude is
18.36 with a 1-sigma error of about 0.15. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.14. 

Burst Advocate for this burst is W. H. Baumgartner (wayne AT milkyway.gsfc.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.)

P. Afonso, R. Filgas and J.Greiner (all MPE Garching) report on behalf of
the GROND team:

We observed the field of GRB 100316B (Swift trigger #416103; Baumgartner
et al., GCN #10489
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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 08:17 UT on 16 March 2010, 16 minutes after the
GRB trigger, and are continuing.

We detect a single point source within the UVOT error circle reported by
Baumgartner et al.

Based on the first 4 min of total exposures in g'r', we estimate
preliminary magnitudes (all in AB system) of

g' = 19.2 +- 0.1 mag,

r' = 18.9 +- 0.1 mag,

indicating reshift < 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)=0.14 mag in the direction of the burst (Schlegel et
al. 1998).

J. Haislip, K. Ivarsen, D. Reichart, A. LaCluyze, R. Egger, A. Foster, J. 
Moore, A. Oza, M. Schubel, J. Styblova, A. Trotter, J. A. Crain, and M. 
Nysewander report:

Skynet observed the Swift/BAT localization of GRB 100316B (Baumgartner et 
al., GCN 10489
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 ) with four of the 16" PROMPT telescopes at CTIO beginning 32 
seconds after the trigger (19 seconds after notification) in BVRI.

We detect the afterglow (Baumgartner et al., GCN 10489
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 ) in all filters.

At 34 seconds after the trigger, the afterglow's magnitude was I = 15.61 
+0.16 -0.14 (statistical) +/- 0.20 (systematic; calibrated to 56 USNO B1 
stars).

Between 34 seconds and 2.1 minutes after the trigger, the afterglow faded 
with a power-law index of about -1.0.

Between 2.1 and 35 minutes after the trigger, the afterglow faded more 
slowly, with a power-law index of about -0.3.

Between 35 and 41 minutes after the trigger, the afterglow suddenly 
rebrightened, with a power-law index of about +5.4.

Between 41 and 49 minutes after the trigger, the afterglow faded with a 
power-law index of about -0.6.

Between 49 and 95 minutes after the trigger, the afterglow faded more 
rapidly, with a power-law index of about -1.7.

At 95 minutes after the trigger, the afterglow's magnitude was R = 19.51 
+0.33 -0.26 (statistical) +/- 0.28 (systematic; calibrated to 40 USNO B1 
stars).

Skynet's most recent BVRI light curve, calibrated to USNO B1 and NOMAD 
stars, can be found here:

http://skynet.unc.edu/grb/grb100316b.png

S.D. Vergani (APC/Univ. Paris 7), P. D'Avanzo (INAF-OAB), D. Malesani, 
J.P.U. Fynbo, J. Hjorth (DARK/NBI), A.J. Levan (Univ. Warwick), H. Flores
(Paris Obs.), N.R. Tanvir (Univ. Leicester), P. Goldoni (APC/Univ. Paris 7
and SAp/CEA), L.A. Antonelli (INAF-OAR) report on behalf of a larger
collaboration:

We observed the optical afterglow of GRB 100316B (Baumgartner et al., GCN
10489
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 ; Afonso et al. GCN 10493
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 ; Haislip et al., GCN 10494
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 ) with the VLT
equipped with the X-shooter spectrograph. Observations started on 2010 Mar
16 at 08:43 UT (~ 40 minutes after the GRB), for a total exposure time of
4x600 s. From the acquisition image, we estimate for the optical afterglow
a magnitude of R = 18.2 +- 0.1 (calibrated against USNO-B1.0).
In a preliminary analysis of the specta, we clearly detect the continuum
in the UVB, VIS, and NIR arms, corresponding to the wavelength range
3200-24000 AA. We detect several absorption features, among these
FeII(1608), AlII(1670), AlIII(1854), FeII(2344), FeII(2374), FeII(2382),
MgII(2796, 2803) and MgI(2852) at a common redshift of z=1.180. The lack
of Lyman forest lines down to ~3200 AA sets an upper limit to the redshift
z<1.6. We also find evidence for FeII lines and MgII doublet from a strong
intervening systems at z=1.063.

We caution that the above analysis (particularly the wavelength solution)
is preliminary. Reduction with updated calibrations is underway. We would
like to thank the ESO staff for their kind assistance, in particular Linda
Schmidtobreick, Christophe Martayan, Patricia Guajardo and Manuel
Olivares.

C. B. Markwardt (GSFC/UMD), S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC),
J. R. Cummings (GSFC/UMBC), N. Gehrels (GSFC), H. A. Krimm (GSFC/USRA),
D. M. Palmer (LANL), 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-61 to T+242 sec from recent telemetry downlinks,
we report further analysis of BAT GRB 100316B (trigger #416103)
(Baumgartner, et al., GCN Circ. 10489
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 ).  The BAT ground-calculated position is
RA, Dec = 163.499, -45.464 deg, which is 
   RA(J2000)  =  10h 53m 59.8s 
   Dec(J2000) = -45d 27' 49.3" 
with an uncertainty of 1.4 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 83%.
 
The mask-weighted light curve shows a FRED peak starting at T-0.3 sec,
peaking at ~T+0.2 sec, and ending at ~T+20 sec.  T90 (15-350 keV) is
3.8 +- 0.5 sec (estimated error including systematics).
 
The time-averaged spectrum from T-0.4 to T+4.0 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
2.23 +- 0.18.  The fluence in the 15-150 keV band is 2.0 +- 0.2 x 10^-07 erg/cm2.
The 1-sec peak photon flux measured from T-0.11 sec in the 15-150 keV band
is 1.3 +- 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/416103/BA/

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

Using 1148 s of XRT Photon Counting mode data and 2 UVOT
images for GRB 100316B, 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 = 163.48865, -45.47244 which is equivalent
to:

RA (J2000): 10h 53m 57.28s
Dec (J2000): -45d 28' 20.8"

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.

S. R. Oates (MSSL-UCL) and W. H. Baumgartner (GSFC/UMBC) report on
behalf of the Swift/UVOT team:

The Swift/UVOT began observerving the field of GRB 100316B 67s
after the BAT trigger (Baumgartner et al., GCN Circ. 10489
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 ). We detect the
optical afterglow in the white, v, b, u and marginally in the uvw1 filter.
The refined UVOT position is RA (J2000)= 163.48809, DEC (J2000)=-45.47265,
which is equivalent to:

     RA (J2000) = 10:53:57.141
    Dec (J2000) = -45:28:21.54

with an estimated uncertainty of 0.52 arcsec (radius, 90% confidence).
This position is consistent with the XRT refined position (Beardmore
et al., GCN Circ 10502
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 ). The marginal detection in the uvw1 filter is
consistent with
the VLT redshift upper limit (D'Avanzo et al., GCN Circ. 10495
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 ).


Preliminary magnitudes and the 3 sigma upper limits are reported below for
individual optical images and summed UV images.

Filter    T_start(s)  T_stop(s)  Exposure      Mag  Err
###########################################################
white     67           216         147         18.36 +/- 0.05
white     857          1007        147         19.15 +/- 0.09
u	  279          528         246         18.44 +/- 0.10
v         1212         1232        19          18.28 +/- 0.46
b         535          555         19          18.97 +/- 0.36
uvw1	  659          1108        58          18.91 +/- 0.36
uvm2      634          1246        66          > 18.59
uvw2      585          1207        78          > 19.13
###########################################################

The above magnitudes are not corrected for the Galactic extinction
corresponding to a non-negligible reddening of E_{B-V} = 0.14 mag
(Schlegel et al.,
1998, ApJS, 500, 525).  The photometry is on the UVOT photometric system
described in Poole et al. (2008, MNRAS, 383,627).

O. M. Littlejohns, P. A. Evans, K. L. Page (U. Leicester) and W. H.
Baumgartner (GSFC/UMBC) report on behalf of the Swift-XRT team:

We have analysed 2.9 ks of XRT data for GRB 100316B (Baumgartner et al.
GCN Circ. 10489
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 ), from 75 s to 40.1 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 10502).

The light curve can be modelled with a broken power-law decay with an
initial decay index of alpha=0.1 (+/-0.2), followed by a break at T~1100
s to a final decay of alpha=1.2 (+0.5, -0.1).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.35 (+0.24, -0.21). The
best-fitting absorption column is 4.1 (+2.7, -1.3) x 10^21 cm^-2, at a
redshift of 1.18 (Vergani et al. GCN Circ 10495
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 ), in addition to the
Galactic value of 9.9 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 3.4 x 10^-11 (5.8 x 10^-11) erg cm^-2 count^-1.

If the light curve continues to decay with a power-law decay index of
1.2, the count rate at T+48 hours will be 6.6 x 10^-4 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.2 x
10^-14 (3.8 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/00416103.

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