GRB 081007

W. H. Baumgartner (GSFC/UMBC), J. R. Cummings (NASA/UMBC),
P. A. Evans (U Leicester), S. D. Hunsberger (PSU),
C. B. Markwardt (CRESST/GSFC/UMD), K. L. Page (U Leicester),
D. M. Palmer (LANL), A. M. Parsons (GSFC), M. Stamatikos (NASA/ORAU),
R. L. C. Starling (U Leicester) and M. C. Stroh (PSU) report on behalf
of the Swift Team:

At 05:23:52 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 081007 (trigger=330856).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 339.976, -40.136 which is 
   RA(J2000) = 22h 39m 54s
   Dec(J2000) = -40d 08' 08"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a double-peaked
structure with a duration of about 25 sec.  The peak count rate
was 1500 counts/sec (15-350 keV), at 1 sec after the trigger. 

The XRT began observing the field at 05:25:32.1 UT, 99.3 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 339.96077, -40.14675 which
is equivalent to:
   RA(J2000)  = 22h 39m 50.58s
   Dec(J2000) = -40d 08' 48.3"
with an uncertainty of 3.1 arcseconds (radius, 90% containment). This
location is 57 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
1.38e+20 cm^-2 (Kalberla et al. 2005). 

The initial flux in the 2.5 s image was 1.53e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 100 seconds with the White (160-650 nm)
filter starting 108 seconds after the BAT trigger. There is a candidate
afterglow in the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	22:39:50.40 = 339.9600
  DEC(J2000) = -40:08:48.8  = -40.1469
with a 1-sigma error radius of about 0.5 arc sec. This position is 2.2 arc sec. 
from the center of the XRT error circle. The estimated magnitude is 17.0 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 0.01. 

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.)

S. Covino, L.A. Antonelli, L. Calzoletti,  S. Campana, G.  
Chincarini,  M.L. Conciatore, S. Cutini,  V. D'Elia,  F. D'Alessio,  
P. D'Avanzo,  F. Fiore, D. Fugazza, P. Goldoni, D. Guetta, C.  
Guidorzi, G.L.  Israel, E. Maiorano, N. Masetti, A. Melandri, E.  
Meurs, L. Nicastro, E. Palazzi, E. Pian, S. Piranomonte, L.  Stella,   
G.  Stratta, G. Tagliaferri, G. Tosti, V.Testa, S.D. Vergani, F.  
Vitali report on  behalf of the REM team:


The robotic 60-cm REM telescope located at La Silla (Chile) observed  
automatically the field of the GRB 081007 (Baumgartner et al. GCN  
8330
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 ) starting about 35s s from the GRB alert (46s from the GRB time)  
with the VRIJHK bands.

We detect a source with coordinates RA,DEC =  
(22:39:50.39,-40:08:49.1) consistent with the UVOT source at H~14.2.  
The source is slowly fading after the first observations.

Further observations are in progress.

B. E. Cobb (UC Berkeley) reports:

Using the ANDICAM instrument on the 1.3m telescope at CTIO, we
obtained optical/IR imaging of the error region of GRB 081007
(GCN 8330
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 , Baumgartner et al.), starting 23 minutes post-burst
in BVRIJHK.

In the R-band, the GRB afterglow (GCN 8330
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 , Baumgartner et al.
& GCN 8331
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 , Covino et al.) appears to fade by approximately 0.6 magnitudes
between 23 minutes and 56 minutes post-burst, indicating
a decay rate of alpha = -0.6 (where afterglow flux is proportional to
t^alpha).

Observations of this burst are continuing.

R.J. Smith, A. Melandri, I.A. Steele, D.F. Bersier, C.G. Mundell,
(Liverpool JMU), A. Gomboc (Ljubljana), C. Guidorzi (INAF-OAB)
on behalf of a larger collaboration reports:

The robotic 2m Faulkes Telescope North located in Hawaii observed
the field of the GRB 081007 (Baumgartner et al. GCN 8330
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 ) starting
automatically as soon as the target rose above the observable horizon
about 17 minutes after the GRB time.

A fading optical source is detected at the XRT position reported by
(Baumgartner et al. GCN 8330
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 ). Over the period from 0.2 - 1 hour after the
burst, the OT is seen to fade in both R and I with a flux power law slope
of -0.7.

Filter  Mag		T_mid[min]
-----------------------------
R       17.9 +/- 0.4	17.50
R       18.6 +/- 0.4	46.66

I       17.1 +/- 0.4	21.55
I       17.6 +/- 0.4	39.38
-----------------------------


The calibration was performed using the R2 and I magnitudes of the
USNOB-1 catalogue field star at 22:39:43 -40:10:15.3 J2000. A equivalent
amount of B-band data in which the source is detected have also been
obtained and are being analysed. Further observations are ongoing.

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

Using 3344 s of XRT Photon Counting mode data and 4 UVOT
images for GRB 081007, 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 = 339.96031, -40.14731 which is equivalent
to:

RA (J2000): 22h 39m 50.48s
Dec (J2000): -40d 08' 50.3"

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
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.

E. Berger (Harvard), D. B. Fox, A. Cucchiara (Penn State), and S. B. Cenko 
(UC Berkeley) report on behalf of a larger collaboration:

"Starting on 2008 October 7.276 UT (73 min after the burst) we observed 
the optical counterpart of GRB 081007 (GCN 8330
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 ) using GMOS on the 
Gemini-South 8-m telescope.  Two 900-sec exposures were obtained covering 
a wavlength range of 4600-7400A.  We identify two absorption features at 
6016.7 and 6070.3 A, as well as a weak emission line at 5700.9 A, which 
correspond to the CaII H&K and [OII]3727 lines at a redshift of 
z=0.5295+/-0.0001."

K.L. Page (U. Leicester) & W.H. Baumgartner (GSFC/UMBC) report on behalf 
of the Swift-XRT team:

We have analysed the XRT data obtained so far for GRB 081007 (Baumgartner 
et al., GCN Circ. 8330
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 ), consisting of 70 s of Windowed Timing (WT) mode 
and 8 ks of Photon Counting mode data. The UVOT-enhanced position was 
given by Beardmore et al. in GCN Circ. 8334
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The X-ray light-curve showed an initial steep decay with alpha = 4.0 +/- 
0.2 until about 230 s after the burst, at which time the slope flattened 
to 0.74 +/- 0.05.

The WT spectrum can be fitted by a steep absorbed power-law, with Gamma = 
3.00 +0.27/-0.24 and NH = (6.6 +/- 1.1)e21 cm^-2 at z = 0.5295 (Berger, 
Fox & Cenko, GCN Circ. 8335
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 ), in addition to the Galactic value (at z=0) 
of 1.4e20 cm^-2. The observed (unabsorbed) flux over this time (105-176 s 
after the trigger) is 3.21e-10 (1.01e-9) erg cm^-2 s^-1.

If the decay continues at a slope of 0.74, the predicted count rate at 24 
hours after the burst is 0.019 count s^-1, corresponding to an observed 
(unabsorbed) flux of 7.6e-13 (1.2e-12) erg cm^-2 s^-1.

The results of the automatic analysis of the XRT data are available at 
http://www.swift.ac.uk/xrt_products

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

J. Wren, W.T. Vestrand, P.R. Wozniak, H. Davis, B. Norman
of Los Alamos National Laboratory report:

The RAPTOR telescope system responded to Swift trigger
330856 (Baumgartner et al., GCN 8330
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 ) under fair observing conditions.
Our narrow-field instruments began observing the location at
05:24:17.13 UTC, 24.4 s after the Swift trigger and during the end
of the Gamma-Ray emitting interval.  We detect the optical counterpart
reported by Baumgartner et al. (GCN 8330
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 ) and Covino et al. (GCN 8331
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 ).
We first detect the counterpart at R~16.4 at approximately 90s after
the Swift trigger time.  The counterpart brightens over the next 50
seconds to magnitude R~15.4 and then begins fading steadily.  Our
unfiltered images were calibrated against the USNO-B1 R-band.  The
following table gives selected observations, not corrected for
extinction, from this event.

t-mid(s)    exp(s)     mag     mag-err
--------------------------------------------
90.31        5.0      16.43    0.22
139.58      10.0      15.38    0.06
367.83      30.0      16.75    0.20
685.42      30.0      17.28    0.19

GRB 081007, Swift-BAT refined analysis

C. M. Markwardt (UMD/GSFC),
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),
K. McLean (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-120 to T+182 sec from the recent telemetry
downlink, we report further analysis of BAT GRB 081007 (trigger
#330856) (Baumgartner et al., GCN Circ. 8330
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 ).  The BAT
ground-calculated position is RA, Dec = 339.963, -40.146 deg which is
     RA(J2000)  =  22h 39m 51.1s
     Dec(J2000) = -40d 08' 44.8"
with an uncertainty of 1.5 arcmin, (radius, sys+stat, 90%
containment).  The partial coding was 28%.

The mask-weighted light curve shows a large peak of approximately 9
seconds duration at T+0.  This peak is seen mostly below 50 keV; a
cutoff-powerlaw model fit yields an Epeak of approximately 12 keV.
T90 (15-350 keV) is 10.0 +- 4.5 sec (estimated error including
systematics).

The time-averaged spectrum from T-6.9 to T+7.1 sec is best fit by a
simple power-law model.  The power law index of the time-averaged
spectrum is 2.51 +- 0.20.  The fluence in the 15-150 keV band is 7.1
+- 0.8 x 10^-07 erg/cm2.  The 1-sec peak photon flux measured from
T+1.10 sec in the 15-150 keV band is 2.6 +- 0.4 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/330856/BA/

We note that the fluence ratio in a simple power-law fit between the
25-50 keV band and the 50-100 keV band is 1.43.  This fluence ratio is
larger than 1.32 which can be achieved with a Band function of
alpha=-1.0, beta=-2.5, and Epeak=30 keV.  Thus, preliminary analysis
shows that Epeak of the burst is very likely around or below 30 keV.
Therefore the burst can be classified as an X-ray flash (e.g. Sakamoto
et al. ApJ, 679, 570).

B. E. Cobb (UC Berkeley) reports:

Using the ANDICAM instrument on the 1.3m telescope at CTIO, we
continued to obtain optical/IR imaging of the error region of GRB 081007
(GCN 8330
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 , Baumgartner et al.) following GCN 8332
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 .

At a mid-exposure time of 2008-10-07 06:04 UT (40 minutes post-burst),
the GRB afterglow (GCN 8330
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 , Baumgartner et al.) had the following
magnitudes:
B = 19.36 +/- 0.04
R = 18.37 +/- 0.03
I = 17.73 +/- 0.03

Observations were obtained under non-photometric conditions.  These
preliminary magnitudes are calibrated against several USNO-B1.0 stars in
the field, so there is likely an additional photometric calibration error
of ~0.2 magnitudes.

From 23 minutes to 112 minutes post-burst the decay rate of the
afterglow remains at alpha = -0.6 (where afterglow flux is proportional to
t^alpha) in all our optical colors.

Further observations of this burst are planned.

P. J. Brown (PSU) and W. H. Baumgartner (GSFC/UMGC)
report on behalf of the Swift/UVOT team:

The Swift/UVOT observed the field of GRB 081007
starting about 109 s after the BAT detection
(Baumgartner et al., GCN Circ. 8330
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 ).  We report additional
detections and 3 sigma upper limits of the fading afterglow
(where T_start and T_stop represent the
elapsed time since the BAT trigger in seconds).

Filter  T_start(s) T_stop  Exp(s)   Mag or 3 Sigma Limit
uvw2    725        1508    40       >18.8
uvm2    621        1394    59       >18.8
uvw1    646        1419    59       >19.3
u       670        1443    59       18.5 +/- 0.2
b       695        1468    39       19.1 +/- 0.2
v       215        615     394      17.5 +/- 0.1
v       969        1369    393      18.6 +/- 0.1
white   109        209     98       17.0 +/- 0.1
white   862        962     98       18.8 +/- 0.1

The above magnitudes are not corrected for the Galactic extinction
corresponding to a reddening of E(B-V) = 0.015 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).

J. P. West, M. Schubel, J. Haislip, R. Holmes, D. Reichart, M. Nysewander,
A. LaCluyze, K. Ivarsen, J. A. Crain, A. Foster, T. Brennan, R. Rhine, J.
Styblova, A. Trotter, and E. Weaver report:

Skynet observed the localization of GRB 081007 (Baumgartner et al., GCN
8330
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 ) with four of the 16" PROMPT telescopes at CTIO beginning 41 seconds
after the trigger (24 seconds after notification) in UBVRI.

We detect the afterglow (Baumgartner et al., GCN 8330
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 ) in BVRI.  At 121
seconds after the burst, we measure I ~ 14.8 mag (calibrated to 6 USNO B1
stars).

Alicia Soderberg (Harvard/SAO) and Dale Frail (NRAO) report on behalf of 
a larger collaboration:

"We observed the field of GRB 081007 (GCN 8330
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 ) with the Very Large Array 
beginning at Oct 9.19 UT.  At 8.46 GHz we detect a radio source 
coincident with the optical afterglow position at coordinates (J2000):

RA =  22 39 50.3882   +/-  0.0009
Dec = -40 08 49.18    +/-  0.03

with flux density 0.32 +- 0.03 mJy.  Further observations are scheduled.

The National Radio Astronomy Observatory is a facility of the National
Science Foundation operated under cooperative agreement by Associated
Universities, Inc."

E. Bissaldi, S. McBreen (MPE) and V. Connaughton (UAH) 
report on behalf of the Fermi GBM Team: 


"At 05:21:51 UT on 7 October 2008, the Fermi Gamma-ray Burst Monitor
triggered on a very weak burst GRB 081007B (trigger 245049712/081007224).
The duration of this event is about 0.5 s but due to the lack of counts 
it was not possible to accurately localize it or perform spectral 
analysis. 

121 seconds later, at 05:23:52 UT, Swift/BAT triggered on GRB 081007
(Baumgartner et al., GCN 8330
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 ). This event was also detected 
by GBM. The angle from the Fermi LAT boresight to the Swift position 
is 116 degrees. Based on the detector geometry GRB081007 does
not appear to be related to GRB 081007B.

GRB 081007 is single peaked with an estimated T90 (25-900 keV) 
of about 12 s. The time-averaged spectrum from T0+0.6 s to T0+6.8 s 
relative to the Swift trigger is adequately fit by a power law with 
an index of -2.1 +/- 0.1, in agreement with the Swift-BAT refined 
analysis (Markwardt et al., GCN 8338
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 ). It can also be fit 
with a power law function with an exponential high energy cutoff.  
The cutoff energy, parameterized as Epeak, is 40 +/- 10 keV.
In this case the power law index is poorly constrained (-1.4 +/- 0.4).

The fluence (25-900 keV) for the power law model is  
(1.2 +/- 0.1)E-06 erg/cm2. 
The 1 sec peak photon flux measured starting from T0+0.9 in 
the 25-900 keV band is 2.2 +/- 0.2 ph/s/cm2.


The spectral analysis results presented above are preliminary;
the final results will be published in the Fermi GBM GRB Catalog."

Alicia Soderberg, Edo Berger (Harvard CfA), and Derek Fox (PSU) report on
behalf of a larger collaboration:

"We imaged the optical counterpart of GRB 081007 (Baumgartner et al., GCN
8330
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 ) with Gemini-S/GMOS in the griz filters on three separate epochs: Oct
24.02, Nov 2.01, and Dec 5.04 UT corresponding to 16.80, 25.79, and 58.82
days after the burst, respectively.

Using the ISIS image subtraction package and adopting our third epoch as a
template image, we find a residual source in the first and second epochs
at a position coincident with that of the early afterglow.  A comparison
of the residual flux indicates that the transient increased by roughly 0.4
mag in i-band between the first and second epochs and then subsequently
faded.  We attribute this photometric evolution to a supernova component,
in line with the recent report of spectroscopic evidence for an underlying
Type Ibc supernova (Della Valle et al., CBET 1602).

Accounting for a redshift of z=0.53 (Berger et al., GCN 8335
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 ), the
k-corrected absolute magnitude of the object in our second epoch i-band
image is fainter than that SN 1998bw at the same (rest-frame) time.

Further analysis is on-going."

Aquib Moin (Curtin Institute of Radio Astronomy / Australia Telescope
National Facility), Steven Tingay (Curtin Institute of Radio Astronomy),
Chris Phillips (Australia Telescope National Facility), Gregory Taylor
(University of New Mexico), Mark Wieringa (Australia Telescope National
Facility) and Ralph Martin (Perth Observatory) report:

We observed the SWIFT-UVOT position of the GRB081007 afterglow (GCN
8330
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 ) at 4.800 and 4.928 GHz with the Australia Telescope Compact Array
(ATCA) between 01:00:00 UT and 22:00:00 UT on February 21/22, 2009.

We did not detect a radio source at the position of the GRB081007
afterglow (GCN 8330
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 ). The data at 4.800 and 4.928 GHz were merged and
the radio flux density at the GRB optical afterglow position found out
to be -0.215 +/- 0.006 mJy/beam (1-sigma).

The Australia Telescope Compact Array (/ Parkes telescope / Mopra
telescope / Long Baseline Array) is part of the Australia Telescope
which is funded by the Commonwealth of Australia for operation as a
National Facility managed by CSIRO.

See the 4.800 & 4.928 GHz combined image at:

http://cira.ivec.org/dokuwiki/doku.php/grb/grb081007/grb081007_field_image