GRB 080906

L. Vetere (PSU), S. D. Barthelmy (GSFC), P. J. Brown (PSU),
D. N. Burrows (PSU), M. M. Chester (PSU), N. Gehrels (NASA/GSFC),
S. T. Holland (CRESST/USRA/GSFC), 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),
D. M. Palmer (LANL), J. L. Racusin (PSU),
B. Sbarufatti (INAF-IASFPA), M. Stamatikos (NASA/ORAU),
G. Tagliaferri (INAF-OAB), E. Troja (INAF-IASFPA) and
T. N. Ukwatta (GSFC/GWU) report on behalf of the Swift Team:

At 13:33:16 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 080906 (trigger=323984).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 227.890, -80.486 which is 
   RA(J2000) = 15h 11m 34s
   Dec(J2000) = -80d 29' 09"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed one major peak
of duration ~50 sec, with possible smaller peaks.  The peak count rate
was ~2000 counts/sec (15-350 keV), at ~10 sec after the trigger. 

The XRT began observing the field at 13:34:27.6 UT, 71.3 seconds after
the BAT trigger. XRT found a bright, fading, uncatalogued X-ray source located
at RA, Dec 228.0514, -80.5183 which is equivalent to:
   RA(J2000)  = 15h 12m 12.3s
   Dec(J2000) = -80d 31' 05.8"
with an uncertainty of 4.7 arcseconds (radius, 90% containment). This
location is 150 arcseconds from the BAT onboard position, within the
BAT error circle. No event data are yet available to determine the
column density using X-ray spectroscopy. 


UVOT took a finding chart exposure of 100 seconds with the White (160-650 nm)
filter starting 82 seconds after the BAT trigger. There is a candidate
afterglow in the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =  15:12:10.63 = 228.0443
  DEC(J2000) = -80:31:04.1  = -80.5178
with a 1-sigma error radius of about 0.5 arc sec. This position is 4.6 arc sec. 
from the center of the XRT error circle. The estimated magnitude is 19.7 with a
1-sigma error of about 0.5 mag. No correction has been made for the expected
extinction corresponding to E(B-V) of 0.30.

GRB 080906: GROND observations of the optical/NIR afterglow 

P.Afonso, T. Kruehler, J. Greiner, C.Clemens, A. Yoldas (all MPE Garching)
and A. Kuepcue Yoldas (ESO) report on behalf of the GROND team: 

We observed the field of GRB 080906 (Swift trigger 323984; L. Vetere et
al., GCN #8186
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 ), simultaneously in g'r'i'z'JHK with GROND (Greiner et al.
2008, PASP 120, 405) mounted at the 2.2m ESO/MPI telescope at La Silla 
Observatory (Chile). 

Observations started at 00:12 UT on September 7th, 2008, 10.7 hrs after
the GRB trigger.We detect the optical afterglow reported by  L. Vetere et
al. (GCN #8186
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 ) at a position consistent with the UVOT detection in all
bands, implying a redshift smaller than 3.5. 

Preliminary photometry yields the following magnitudes calibrated against
USNO-B1 and 2MASS field stars in stacked images: 

Band  Exp(min)   Mag 

g'    24         22.2 +- 0.1
r'    24         21.4 +- 0.1
i'    24         21.2 +- 0.1
z'    24         21.2 +- 0.1
J     20         20.2 +- 0.1
H     20         19.2 +- 0.1
K     20         18.5 +- 0.1 

not corrected for the expected Galactic foreground reddening of E(B-V) =
0.30 mag (Schlegel et al., 1998).

M. Stamatikos (GSFC/ORAU), 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), K. McLean (GSFC/UMD),
D. Palmer (LANL), T. Sakamoto (GSFC/UMBC), G. Sato (ISAS),
J. Tueller (GSFC), T. Ukwatta (GWU), L. Vetere (PSU)
(i.e. the Swift-BAT team):
 
Using the data set from T-239 to T+963 sec from recent telemetry downlinks,
we report further analysis of BAT GRB 080906 (trigger #323984)
(Vetere, et al., GCN Circ. 8186
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 ).  The BAT ground-calculated position is
RA, Dec = 228.055, -80.540 deg, which is 
   RA(J2000)  =  15h 12m 13.3s 
   Dec(J2000) = -80d 32' 24.9" 
with an uncertainty of 1.7 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 58%.
 
The mask-weighted light curve shows a roughly symmetrical peak starting
at ~T-100 sec, peaking around T+10 sec, and tailing off to about T+100 sec.
T90 (15-350 keV) is 147 +- 21 sec (estimated error including systematics).
 
The time-averaged spectrum from T-105 to T+104 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.59 +- 0.09.  The fluence in the 15-150 keV band is 3.5 +- 0.2 x 10^-6 erg/cm2.
The 1-sec peak photon flux measured from T-0.06 sec in the 15-150 keV band
is 1.0 +- 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/323984/BA/

L. Vetere, J.L. Racusin, C. Pagani (PSU), J.P. Osborne, A.P. Beardmore,
P.A. Evans and M.R. Goad (U. Leicester)  report on behalf of the Swift-XRT team:

The XRT began observing the field of GRB 080906 (trigger=323984; Vetere et al.,
GCN Circ 8186
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 ) at 13:34:27.6 UT, 71.3 seconds after the BAT trigger.

Using 1224 s of XRT Photon Counting mode data and 2 UVOT images, we find an
astrometrically corrected X-ray position (using the XRT-UVOT alignment and
matching to the USNO-B1 catalogue): RA, Dec = 228.04211, -80.51787 which is
equivalent to:

 RA (J2000): 15 12 10.11
 Dec (J2000): -80 31 04.3

 with an uncertainty of 1.8 arcsec (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 position is 80 arcsec away from the ground-processed BAT position given in
GCN Circ. 8196
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  (Stamatikos et al.) inside BAT error circle.


The WT mode lightcurve shows a number of flares, with the strongest peaking
around
186, 598 and 907 s after the trigger. The PC data in later orbits  shows some
flaring activity and decays as a single power-law  with alpha = 1.25 +/- 0.11.

The WT spectrum of the first 80 s (before the first peak) can be fitted with a
power-law of Gamma = 1.82 +/-0.08 and absorption consistent with the Galactic
value
along the line of sight (9.7x10^20 cm^-2). The observed (unabsorbed) flux over
this
time is 1.98 (2.7)x10^-9 erg cm^-2 s^-1. A spectrum extracted from the PC mode
data
(11.6-75.6 ks after the trigger) can be fitted with a power-law of Gamma = 2.3
+0.1/-0.2
and total NH = (1.7 +/- 0.5)x10^22 cm^-2, slightly in excess of the Galactic
value.
The observed (unabsorbed) flux over this time is 1.7 (3.1)x10^-11 erg cm^-2
s^-1.

If the decay continues with alpha = 1.25, the predicted count-rate at 48 hours
is
0.007 count s^-1, corresponding to an observed (unabsorbed) flux of 2.6
(4.8)x10^-13
erg cm^-2 s^-1.


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

S. T. Holland (CRESST/USRA/GSFC) & L. Vetere (PSU) report on behalf of
the Swift/UVOT team:

       The Swift/UVOT observed the field of GRB 080906 starting 62 s
after the BAT trigger (Vetere et al., 2008, GCNC. 8186).  Settled
observations started at T + 82 s.  We detect the optical afterglow
(Vetere et al., GCNC 8186) in the v, b, u, and white filters.  The
refined UVOT source position is

  RA (J2000.0) =  15:12:10.65
Dec (J2000.0) = -80:31:03.2

with an estimated uncertainty of 0.64 arcsec (radius, 90% confidence).

     Magnitudes and upper limits are reported below.

Filter   TSTART   TSTOP   Exposure     Mag  Err  Comment
  white       82     182         98   19.71 0.15
    v        188     588        393   18.61 0.12
    b        668    6204        216   21.02 0.44
    u        643    5998        236   20.37 0.34
   uvw1      619    5793        236  >20.9        2-sigma upper limit
   uvm2      594    5588        236  >20.5        2-sigma upper limit
   uvw2      698    6614        235  >21.1        2-sigma upper limit

The above magnitudes are not corrected for the Galactic extinction
corresponding to a reddening of E_{B-V} = 0.30 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).

      The detection in the u filter, coupled with the non-detections in
the uvw1, uvm2, and uvw2 filters, are consistent with a redshift of
approximately z < 2.6.  The power-law decay index in the v band
between 338 and 6719 s is -0.50 +/- 0.04.

Please update my e-mail address in your Address Book to <Stephen.T.Holland@nasa.gov 
 >.
/
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Alejandro Ferrero (UCD School of Physics), John French (UCD School of
 Physics, DARK/NBI), Gary Melady (UCD School of Physics) report on
 behalf of a larger collaboration:
 
 The Watcher 40 cm robotic telescope at Boyden Observatory, South
 Africa, began imaging the field of GRB 080906 (Vetere et al. GCN 8186
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 )
 at 16:51:15.5 UT, 3.6 h after the Swift triggger. We do not detect the
 afterglow reported by Holland et al. (GCN 8198
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 ) and Afonso et al. (GCN
 8194
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 ). Our first 120s exposure has a limiting magnitude of 16.9, while
 a stacked sequence of 10 120s images, with an exposure mid-time of 3.8
 hrs after the trigger, results in a limiting magnitude of 18.4.
 Unfiltered magnitudes are transformed to R-band by comparison with the
 USNO-B1 catalog.