GRB 080916B

G. Stratta (ASDC), S. D. Barthelmy (GSFC),
A. P. Beardmore (U Leicester), P. A. Evans (U Leicester),
N. Gehrels (NASA/GSFC), C. Guidorzi (INAF-OAB),
S. T. Holland (CRESST/USRA/GSFC), E. A. Hoversten (PSU),
S. D. Hunsberger (PSU), J. A. Kennea (PSU),
H. A. Krimm (CRESST/GSFC/USRA), J. Mao (INAF-OAB),
C. B. Markwardt (CRESST/GSFC/UMD), F. E. Marshall (NASA/GSFC),
P. T. O'Brien (U Leicester), S. R. Oates (UCL-MSSL),
J. P. Osborne (U Leicester), K. L. Page (U Leicester),
M. Perri (ASDC), B. Preger (ASDC), R. L. C. Starling (U Leicester),
M. C. Stroh (PSU) and T. N. Ukwatta (GSFC/GWU) report on behalf of
the Swift Team:

At 14:44:47 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 080916B (trigger=324907).  The BAT on-board calculated location is 
RA, Dec 163.624, +69.044 which is 
   RA(J2000) = 10h 54m 30s
   Dec(J2000) = +69d 02' 39"
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 50 sec.  The peak count rate
was ~750 counts/sec (15-350 keV), at ~0 sec after the trigger. 

Due to an observing constraint, Swift will not slew until 
T0+18.9 minutes. There will be no XRT or UVOT data until
this time. 

Burst Advocate for this burst is G. Stratta (giulia.stratta AT asdc.asi.it). 
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.)

G. Stratta, M. Perri, P. Evans report on behalf of
the Swift-XRT Team:

The XRT began observing the field at 15:07:22.2 UT, 1354.9 seconds
after the BAT trigger. Using promptly downlinked data we find an
uncatalogued X-ray source located at RA, Dec 163.66560, 69.06574 which
is equivalent to:
   RA(J2000)  = 10h 54m 39.74s
   Dec(J2000) = +69d 03' 56.7"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 94 arcseconds from the BAT onboard position, within the BAT
error circle.

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of
1.8e+20 cm^-2 (Kalberla et al. 2005).

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

Using 1049 s of XRT Photon Counting mode data and 1 UVOT
images for GRB 080916B, 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 = 163.66512, +69.06545 which is equivalent
to:

RA (J2000): 10h 54m 39.63s
Dec (J2000): +69d 03' 55.6"

with an uncertainty of 5.1 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
http://www.swift.ac.uk/xrt_positions/Goad.pdf), the current algorithm is an
extension of this method.

This circular was automatically generated, and is an official product of the
Swift-XRT team.

S. D. Barthelmy (GSFC), W. Baumgartner (GSFC/UMBC), J. Cummings (GSFC/UMBC),
E. Fenimore (LANL), N. Gehrels (GSFC), H. Krimm (GSFC/USRA),
C. Markwardt (GSFC/UMD), D. Palmer (LANL), T. Sakamoto (GSFC/UMBC),
G. Sato (ISAS), M. Stamatikos (GSFC/ORAU), G. Stratta (ASDC),
J. Tueller (GSFC), T. Ukwatta (GWU)
(i.e. the Swift-BAT team) report:
 
Using the data set from T-239 to T+617 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 080916B (trigger #324907)
(Strata, et al., GCN Circ. 8240
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 ).  The BAT ground-calculated position is
RA, Dec = 163.632, 69.061 deg, which is 
   RA(J2000)  =  10h 54m 31.6s 
   Dec(J2000) = +69d 03' 38.2" 
with an uncertainty of 2.1 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 53%.
 
The mask-weighted light curve shows a weak FRED-like peak starting
at ~T-4 sec, peaking at ~T+3 sec, and ending at ~T+50 sec.  There is a
possibility of very weak emission lasting out to at least T+350 sec,
at which time the spacecraft slewed away fro the burst location.
T90 (15-350 keV) is 32 +- 8 sec (estimated error including systematics).
 
The time-averaged spectrum from T-3.3 to T+35.7 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.49 +- 0.23.  The fluence in the 15-150 keV band is 6.3 +- 0.9 x 10^-7 erg/cm2.
The 1-sec peak photon flux measured from T+3.30 sec in the 15-150 keV band
is 0.6 +- 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/324907/BA/

E. A. Hoversten (PSU) and G. Stratta (ASDC) report on behalf of the
Swift UVOT team:

The Swift/UVOT began settled observations of the field of
GRB 080916B, 1359s after the BAT trigger (Stratta et al., GCN 8240
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 ).
There is a possible detection of an afterglow in the UVOT white filter
at:

     RA (J2000)  10:54:39.78
    Dec (J2000)  69: 3:57.9

with an estimated uncertainty of 0.8 arcsec (radius, 90% confidence).
This position is consistent with the UVOT enhanced XRT position (Evans
et al., GCN 8247
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 ).  The afterglow candidate does not appear in the DSS
images, and photometry from the second orbit of Swift observations
suggests that it has faded.

UVOT magnitudes and 3-sigma upper limits are reported in the following
table:

Filter   T_start   T_stop   Exp(s)  Mag (3-sigma upper limit)
-----------------------------------------------------
white    1359.0    1458.8    98.2   20.50 +/- 0.21
white    7826.0    8025.7   196.6     > 21.27
v        1340.9    2920.7   342.1     > 19.62
b        1516.3    2676.5   155.6     > 20.32
u        1491.6    2651.7   155.6     > 19.91
uvw1     1467.3    2627.3   155.5     > 19.91
uvm2     1604.9    3076.8   285.0     > 20.06
uvw2     1556.0    2716.0   155.5     > 19.91

The quoted upper limits have not been corrected for the expected
Galactic extinction along the line of sight of E_{B-V} = 0.02 mag.
All photometry is on the UVOT flight system described in Poole et al.
(2008, MNRAS, 383, 627).

G. Stratta, M. Perri, B. Preger (ASDC) report on behalf of the
Swift-XRT team:

We have analyzed the first two orbits of Swift XRT data of
GRB 080916B (Stratta et al., GCN Circ. 8242
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 ), consisting of
2.8 ks of Photon Counting data collected starting
from 22.8 min after the burst.

During the XRT observation, from T0+1.4 ks to T0+8.5 ks,
the source shows a fading behaviour from 0.08 cts/s
to 0.02 cts/s. Assuming a power law model, the best fit decay index is
alpha = 1.0 � 0.2. At this rate we predict a count rate
level of 1.7 x 10-3 cts/s one day after the trigger.

The 0.3-10 keV extracted spectrum (2.8 ks of integration time)
is consistent with an absorbed power law model.
Fixing the column density to the Galactic value
(NH = 1.8 x 1020 cm-2 from Kalberla et al. 2007)
the best fit photon index is 2.0 � 0.5.

The observed 0.3-10.0 keV flux is (1.0+/-0.6) x 10-12 erg cm-2 s-1
which corresponds to an unabsorbed flux of 1.1 x 10-12  erg cm-2 s-1.
The count to observed flux conversion factor is 6.6 x 10-11 erg cm-2
count-1.

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