GRB 091104

P.A. Curran (MSSL-UCL), W. H. Baumgartner (GSFC/UMBC),
A. P. Beardmore (U Leicester), S. Campana (INAF-OAB),
P. A. Evans (U Leicester), C. Guidorzi (U Ferrara),
J. A. Kennea (PSU), V. Mangano (INAF-IASFPA), J. Mao (INAF-OAB),
P. T. O'Brien (U Leicester), K. L. Page (U Leicester),
D. M. Palmer (LANL), A. Rowlinson (U Leicester) and M. H. Siegel (PSU)
report on behalf of the Swift Team:

At 08:49:22 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 091104 (trigger=374875).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 208.742, +47.398 which is 
   RA(J2000) = 13h 54m 58s
   Dec(J2000) = +47d 23' 55"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is usual for an image trigger, there
is no obvious variation in the immediately available light curve. 

The XRT began observing the field at 08:51:34.2 UT, 131.5 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 208.75600, 47.41126 which
is equivalent to:
   RA(J2000)  = 13h 55m 1.44s
   Dec(J2000) = +47d 24' 40.5"
with an uncertainty of 4.0 arcseconds (radius, 90% containment). This
location is 58 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
1.72e+20 cm^-2 (Kalberla et al. 2005). 

The initial flux in the 2.5 s image was 1.85e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with
the White filter starting 139 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 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 0.01. 

Burst Advocate for this burst is P.A. Curran (pac AT mssl.ucl.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.)

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

Using 677 s of XRT Photon Counting mode data and 1 UVOT
images for GRB 091104, 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 = 208.75624, +47.41054 which is equivalent
to:

RA (J2000): 13h 55m 1.50s
Dec (J2000): +47d 24' 37.9"

with an uncertainty of 1.6 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.

P.A. Curran (MSSL-UCL) reports on behalf of the Swift/UVOT team:


The Swift/UVOT began settled observations of the field of GRB 091104 139s 
after the BAT trigger (Curran et al., GCN 10130
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 ). No optical afterglow 
consistent with the enhanched XRT position (Osborne et al., GCN 10131
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 ) is 
detected in the initial UVOT exposures. Preliminary 3-sigma upper limits 
using the UVOT photometric system (Poole et al. 2008, MNRAS, 383, 627) for 
the finding charts (FC) and subsequent exposures are:


Filter    T_start(s)   T_stop(s)    Exp(s)    Mag
-------------------------------------------------------
white     139          289          147       >20.56 (FC)
u         297          547          246       >19.69 (FC)
white     139          993          301       >20.84
v         628          6155         87        >18.53
b         553          746          39        >18.88
u         297          721          265       >19.74
w1        677          870          39        >18.45
m2        652          845          39        >18.26
w2        776          4636         151       >19.71

The values quoted above are not corrected for the Galactic extinction due 
to the reddening of E(B-V) = 0.01 in the direction of the burst (Schlegel 
et al. 1998).

A. Rowlinson (U Leicester) and P.A. Curran (MSSL-UCL) report on behalf
of the Swift-XRT team:

We have analysed 2.7 ks of XRT data for GRB 091104 (Curran et al. GCN
Circ. 10130
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 ), from 137 s to 16.5 ks after the BAT trigger. The data
comprise 162 s in Windowed Timing (WT) mode with the remainder in Photon
Counting (PC) mode. The enhanced XRT position for this burst was given
by Osborne et al. (GCN. Circ 10131). The light curve can be modelled
with a series of power-law decays. The initial decay index is alpha=0.55
(+0.24, -0.27). At T+226.54 s the decay steepens to an alpha of 4.86
(+0.35, -0.29) before breaking again at T+524.02 s to a final decay with
index alpha=1.48 (+1.19, -0.29).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.298 (+0.103, -0.096). The
best-fitting absorption column is 1.09 (+0.20, -0.19) x 10^21 cm^-2, in
excess of the Galactic value of 1.7 x 10^20 cm^-2 (Kalberla et al.
2005). The PC mode spectrum has a photon index of 2.31 (+0.22, -0.15)
and a best-fitting absorption column consistent with the Galactic value.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum is 3.1 x 10^-11 (3.4 x 10^-11) erg cm^-2
count^-1.

If the light curve continues to decay with a power-law decay index of
1.48, the count rate at T+24 hours will be 1.9 x 10^-4 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.9 x
10^-11 (6.5 x 10^-11) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00374875.
This circular is an official product of the Swift-XRT team.

J. R. Cummings (GSFC/UMBC), S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC),
P.A. Curran (MSSL-UCL), E. E. Fenimore (LANL), N. Gehrels (GSFC),
H. A. Krimm (GSFC/USRA), C. B. Markwardt (GSFC/UMD), D. M. Palmer (LANL),
A. M. Parsons (GSFC), T. Sakamoto (GSFC/UMBC), G. Sato (ISAS),
M. Stamatikos (GSFC/ORAU), J. Tueller (GSFC), T. N. Ukwatta (GWU)
(i.e. the Swift-BAT team):
 
Using the data set from T-239 to T+903 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 091104 (trigger #374875)
(Curran, et al., GCN Circ. 10130
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 ).  The BAT ground-calculated position is
RA, Dec = 208.723, 47.391 deg, which is 
   RA(J2000)  =  13h 54m 53.6s 
   Dec(J2000) = +47d 23' 25.8" 
with an uncertainty of 2.6 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 82%.
 
The mask-weighted light curve shows a sharp rise at ~T+2, a rather flat top,
and ending at ~T+120 sec.  T90 (15-350 keV) is 99.3 +- 20 sec
(estimated error including systematics).
 
The time-averaged spectrum from T-1.5 to T+120.5 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.74 +- 0.24.  The fluence in the 15-150 keV band is 7.7 +- 1.2 x 10^-7 erg/cm2.
The 1-sec peak photon flux measured from T+83.34 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/374875/BA/