GRB 090831C

P. Schady (MSSL-UCL), S. D. Barthelmy (GSFC),
W. H. Baumgartner (GSFC/UMBC), A. P. Beardmore (U Leicester),
J. R. Cummings (NASA/UMBC), P. A. Evans (U Leicester),
C. Guidorzi (U Ferrara), S. T. Holland (CRESST/USRA/GSFC),
J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA),
C. B. Markwardt (CRESST/GSFC/UMD), F. E. Marshall (NASA/GSFC),
C. Pagani (PSU), K. L. Page (U Leicester), D. M. Palmer (LANL),
A. Rowlinson (U Leicester), M. H. Siegel (PSU),
M. Stamatikos (OSU/NASA/GSFC) and M. A. Stark (PSU) report on behalf
of the Swift Team:

At 21:30:25 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 090831C (trigger=361489).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 108.278, -25.100 which is 
   RA(J2000)  =  07h 13m 07s
   Dec(J2000) = -25d 06' 00"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a single peak
with a duration of about 10 sec.  The peak count rate
was ~800 counts/sec (15-350 keV), at ~3 sec after the trigger. 

The XRT began observing the field at 21:32:23.5 UT, 117.5 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source located at RA, Dec 108.29307, -25.11934 which
is equivalent to:
   RA(J2000)  = 07h 13m 10.34s
   Dec(J2000) = -25d 07' 09.6"
with an uncertainty of 3.8 arcseconds (radius, 90% containment). This
location is 85 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
3.24e+21 cm^-2 (Kalberla et al. 2005). 


UVOT took a finding chart exposure of 150 seconds with the White filter
starting 120 seconds after the BAT trigger. No credible afterglow candidate has
been found in the initial data products. Results from the 2.7'x2.7' sub-image
are not available at this time. The coverage of the XRT error circle by the
8'x8' region for the list of sources generated on-board is uncertain because
the large number of sources filled the available telemetry. No correction has
been made for the expected extinction corresponding to E(B-V) of 0.38. 




Burst Advocate for this burst is P. Schady (ps 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.)

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

Using 3607 s of XRT Photon Counting mode data and 3 UVOT
images for GRB 090831C, 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 = 108.29453, -25.11859 which is equivalent
to:

RA (J2000): 07h 13m 10.69s
Dec (J2000): -25d 07' 06.9"

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, arXiv:0812.3662).

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

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

We have analysed 8.4 ks of XRT data for GRB 090831C (Schady et al. GCN
Circ. 9854
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 ), from 121 s to 21.8 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 9855).

The light curve has two flares at T+186 s and T+440 s, while the
underlying decay can be modelled with a power-law decay with a decay
index of alpha=0.83 (+0.17, -0.11).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.84 (+0.21, -0.15). The
best-fitting absorption column is 3.254 (+1.001, -0.014) x 10^21 cm^-2,
consistent with the Galactic value of 3.2 x 10^21 cm^-2 (Kalberla et al.
2005). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum is 4.9 x 10^-11 (7.2 x 10^-11) erg
cm^-2 count^-1.

If the light curve continues to decay with a power-law decay index of
0.83, the count rate at T+24 hours will be 0.0029 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.4 x
10^-13 (2.1 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/00361489.

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

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

The Swift/UVOT began settled observations of the field of GRB 090831C 121s 
after the BAT trigger (Schady et al., GCN Circ 9854
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 ). No new source is 
detected within the XRT error circle (Evans et al., GCN Circ 9855
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 ) in any 
of the individual or combined UVOT filters.

The 3-sigma upper limits of detecting a source at the position of the 
X-ray afterglow in the first white band finding chart (fc) observation and 
in consecutive coadded observations for each of the UVOT filters are as 
follows:

Filter	T_start(s) T_stop(s) Exp(s) Mag 3-sig UL
wh (fc)    121       270     147      > 20.78
wh         561       1183    206      > 20.73
vv         610       1060    58       > 18.47
bb         536       1159    58       > 19.30
uu         279       1134    285      > 19.95
w1         660       1109    58       > 18.74
m2         635       1084    58       > 18.48
w2         586       1209    78       > 18.95


The values quoted above are not corrected for the Galactic extinction due 
to the large reddening of E(B-V) = 0.38 in the direction of the burst 
(Schlegel et al., 1998). The photometry is on the UVOT photometric system 
described in Poole et al. (2008, MNRAS, 383, 627).

F. Olivares, T. Kruehler and J. Greiner (all MPE Garching) report on behalf 
of the GROND team:

We observed the field of GRB 090831C (Swift trigger #361489; Schady et al., 
GCN #9854
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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 LaSilla 
Observatory (Chile).

Observations started at 08:07 UT on September 1st, 2009, 10.6 hr after the 
burst. At this time the position of the burst was becoming visible above 
the pointing constraints of the telescope. Observations suffer from high 
airmass and large seeing of around 2". In stacked images of 74 min total 
integration time in griz and 60 min in JHK, we do not detect any object 
within the enhanced XRT error circle (Evans et al., GCN #9855
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 ), down to the 
following limiting magnitudes (all in the AB system):

g' > 23.5
r' > 24.0
i' > 23.3
z' > 22.9
J > 21.4
H > 21.1
K > 20.0

These upper limits were derived using the GROND zeropoints and 2MASS field 
stars as reference and are not corrected for significant Galactic 
foreground extinction corresponding to a reddening of E(B-V)=0.38 mag 
(Schlegel et al., 1998).

C. B. Markwardt (GSFC/UMD), S. D. Barthelmy (GSFC),
W. H. Baumgartner (GSFC/UMBC), J. R. Cummings (GSFC/UMBC),
E. E. Fenimore (LANL), N. Gehrels (GSFC), H. A. Krimm (GSFC/USRA),
D. M. Palmer (LANL), A. M. Parsons (GSFC), T. Sakamoto (GSFC/UMBC),
G. Sato (ISAS), P. Schady (MSSL-UCL), M. Stamatikos (OSU),
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 the recent telemetry
downlink, we report further analysis of BAT GRB 090831 (trigger #361489)
(Schady, et al., GCN Circ. 9854
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 ).  The BAT ground-calculated position is
RA, Dec = 108.294, -25.112 deg which is
    RA(J2000)  =  07h 13m 10.6s
    Dec(J2000) = -25d 06' 44.0"
with an uncertainty of 2.1 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 84%.

The mask-weighted light curve shows a single roughly symmetrical peak.
T90 (15-350 keV) is 3.3 +- 1.0 sec (estimated error including systematics).

The time-averaged spectrum from T+0.6 to T+4.9 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.33 +- 0.29.  The fluence in the 15-150 keV band is
1.5 +- 0.3 x 10^-07 erg/cm2.  The 1-sec peak photon flux measured from
T+1.94 sec in the 15-150 keV band is 0.5 +- 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/361489/BA/