GRB 081008

E.S. Rykoff (UCSB), report on behalf of the ROTSE collaboration:

ROTSE-IIIc, located at the H.E.S.S. site at Mt. Gamsberg, Namibia, 
responded to GRB 081008 (Swift trigger 331093). The first image was at 
19:58:51.3 UT, 41.9 s after the burst (8.3 s after the GCN notice time). 
The unfiltered images are calibrated relative to USNO A2.0. We detect a 
14.5 magnitude, brightening source with coordinates:

      18:39:50.0      -57:25:52.0    (J2000), with positional 
uncertainty of 1" or better

start UT    	mag     mlim(of image)
----------------------------------
19:58:51.3     14.5     16.2


This source is not visible in DSS (second epoch), 2MASS or the MPChecker 
database.

A jpeg image is available at 
http://www.rotse.net/images/gsb331093_3c00_img.jpg

Continuing observations are in progress.

J. L. Racusin (PSU), W. H. Baumgartner (GSFC/UMBC), P. J. Brown (PSU),
D. N. Burrows (PSU), P. A. Evans (U Leicester), D. Grupe (PSU),
S. T. Holland (CRESST/USRA/GSFC), E. A. Hoversten (PSU),
J. A. Kennea (PSU), V. La Parola (INAF-IASFPA),
F. E. Marshall (NASA/GSFC), P. T. O'Brien (U Leicester),
J. P. Osborne (U Leicester), A. M. Parsons (GSFC),
M. Stamatikos (NASA/ORAU), R. L. C. Starling (U Leicester),
G. Tagliaferri (INAF-OAB), T. N. Ukwatta (GSFC/GWU) and
L. Vetere (PSU) report on behalf of the Swift Team:

At 19:58:09 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 081008 (trigger=331093).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 279.987, -57.464 which is 
   RA(J2000) = 18h 39m 57s
   Dec(J2000) = -57d 27' 50"
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 150 sec.  The peak count rate
was ~2500 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 19:59:36.5 UT, 87.1 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 279.9587, -57.4312 which is equivalent to:
   RA(J2000)  = 18h 39m 50.08s
   Dec(J2000) = -57d 25' 52.3"
with an uncertainty of 5.3 arcseconds (radius, 90% containment). This
location is 130 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 150 seconds with the White (160-650 nm)
filter starting 96 seconds after the BAT trigger. There is a candidate
afterglow in the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	18:39:50.11 = 279.9588
  DEC(J2000) = -57:25:52.3  = -57.4312
with a 1-sigma error radius of about 1.6 arc sec. This position is 0.3 arcseconds
from the center of the XRT error circle. The estimated magnitude is 15.0 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 0.10. 

Burst Advocate for this burst is J. L. Racusin (racusin AT astro.psu.edu). 
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. D'Avanzo (INAF-OAB), V. D'Elia (INAF-OAR), S. Covino (INAF-OAB) 
report, on behalf of the MISTICI collaboration:

Following the detection of GRB 081008 (Rykoff, GCN 8343
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 ; Racusin et al., 
GCN 8344
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 ), we triggered observations with the FORS2 camera, mounted at 
ESO-VLT. The observations were performed using the 600B grism. We took a 
900 s spectrum starting around Oct 09 at 00:20 UT (about 4.5 hours after 
the burst) and covering the range 380-620 nm.

The non-detection of Ly-alpha sets an upper limit of z ~ 2 to the GRB 
redshift. We detect a double horned absorption feature around lambda 
4600 A which we identify as the MgII doublet at 2796-2803 A. This 
implies a redshift of the absorber of z = 0.64. We also detect two 
absorption features around lambda 4530-4550 A which we identify as FeII 
2586 and FeII 2600 A. We possibly interpret this signature as another 
absorption system at z = 0.75.

However, at the present stage, the most robust redshift range that we 
can set for GRB 081008 is 0.64 < z < 2.

Further analysis is in progress.

We are very grateful for the excellent support of the VLT staff.

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

Starting at 00:20 UT on 2008 October 9 (T0+5h) we observed the optical
counterpart of GRB 081008 (Racusin et al., GCN 8344
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 , Rykoff  et al.,
GCN 83433
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 ) using Gemini-South with the GMOS-South spectrograph (R~1600).
We acquired 2x900s spectra covering the wavelength range 3800A-6700A.

We clearly detect several metal absorption features which we interpret as
SiIV(1393,1402), SiII1526, SiII*1816, CIV(1548,1550), FeII1608, AlII1670,
NiII1703, AlIII(1854,1862), MgI2026 at the common redshift of z = 1.967.

The absorption system identified by D'Avanzo et al. (GCN 8345
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 ) can be
explained by the CIV and SiII/SiII* doublets at this redshift.

We thank the Gemini staff for conducting these observations.

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

Using 8661 s of XRT Photon Counting mode data and 8 UVOT
images for GRB 081008, 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 = 279.95776, -57.43188 which is equivalent
to:

RA (J2000): 18h 39m 49.86s
Dec (J2000): -57d 25' 54.8"

with an uncertainty of 1.4 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.

P. Schady (MSSL-UCL) and J. L. Racusin (PSU) report on behalf of the 
Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 081008 96 s 
after the BAT trigger (Racusin et al., GCN Circ. 8344
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 ), using a newly 
implemented observing sequence. The new sequence takes a 150s white-band 
finding chart exposure, followed by a 250s u-band exposure, after which 
the filter wheel rotates through all filters, taking short 20s exposures.

A new fading source is detected at the ROTSE position (Rykoff et al., GCN 
Circ 8343
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 ) in the white, v, b, u and uvw1 filters, consistent with a� 
redshift of z=1.967 reported by Cucchiara et al., (GCN Circ 8346
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 ). The 
source is decaying at a constant rate of 0.90+/-0.03 for the duration of 
intial UVOT observations, out to ~700s after the BAT trigger. The UVOT 
refined position is
RA(J2000) = 18:39:49.877 (279.95833 deg)
Dec(J2000) = -57:25:52.87 (-57.431111 deg)
with an estimated uncertainty of 0.6 arcsec (radius, 90% confidence).

The detections and 3 sigma upper limits of the fading afterglow are 
reported below, where T_start and T_stop represent the elapsed time since 
the BAT trigger in seconds, and fc indicate finding chart exposures

Filter      T_start(s) T_stop� Exp(s)�� Mag or 3 Sigma Limit
white (fc)   96������ � 246���� 146���� �14.96 +/- 0.01
white��������534��������554���� 19�������15.87 +/- 0.03
u (fc)��� � �254������ �504�� ��246������15.19 +/- 0.02
u����� � � � 658������ �678���� 19���� � 15.81 +/- 0.08
v����� � � � 584��������604�����19����� �15.64 +/- 0.10
b����������� 510������ �530�� ��19�������15.79 +/- 0.05
uvw1�� � � � 633����� � 653�� ��19����� �17.03 +/- 0.22
uvm2�������  608������ �1601����117����� > 19.16
uvw2�� � � � 559��������1717��� 128������> 19.53

The above magnitudes are not corrected for the Galactic extinction 
corresponding to a reddening of E(B-V) = 0.10 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. L. Racusin (PSU) reports on behalf of the Swift-XRT team:

We have analyzed the first 7 orbits of XRT data for GRB 081008 (Racusin et 
al., GCN 8344
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 ), including 315 s of WT data and 6.9 ks of PC data.  The 
UVOT-enhanced position was given by Evans et al. (GCN 8347
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 ).  The 
results of the automatic analysis of the XRT data are available at 
http://www.swift.ac.uk/xrt_products.

The 0.3-10.0 keV lightcurve shows three bright flares superimposed on a 
steep decay, followed by a plateau with a decay index of 0.9 +/- 0.1, 
breaking at 16 +/- 14 ks to a decay index of 2.0 {+1.8,-0.9}.

The WT spectrum, for which changes in the hardness ratio during the flares 
suggests spectral evolution, can be fit by an absorbed power-law, with 
photon index = 1.81 +/- 0.03 and NH = (6.7 +/- 0.8)e21 cm^-2 at z = 1.967 
(Cucchiara et al., GCN 8346
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 ), in addition to the Galactic value of 7.1e20 
cm^-2. The observed (unabsorbed) flux over this time (93-408 s after the 
trigger) is 3.1e-9 (3.9e-9) erg cm^-2 s^-1.

The PC spectrum can be fit by an absorbed power-law, with photon index = 
1.91 +/- 0.07 and NH = (5.9 {+15.1, -5.9})e20 cm^-2 at z = 1.967, 
consistent with the Galactic value. The observed (unabsorbed) flux over 
this time (412 s - 11.9 ks after the trigger) is 3.4e-11 (4.0e-11) erg 
cm^-2 s^-1.

Assuming the X-ray emission continues to decline at the same rate, we 
predict a 0.3-10.0 keV XRT count rate of 3.6e-3 counts/s at T+24hr, which 
corresponds to an observed (unabsorbed) flux of 1.4e-13 (1.7e-13) ergs 
cm^-2 s^-1.

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

P. D'Avanzo (INAF-OAB), V. D'Elia (INAF-OAR), S. Covino (INAF-OAB) 
report, on behalf of the MISTICI collaboration:

We observed the optical afterglow of GRB 081008 (Rykoff, GCN 8343
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 ; 
Racusin et al., GCN 8344
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 ), with the ESO VLT on Oct 09 starting about 4.5 
hours after the burst with UVES. In the UVES spectrum  we clearly detect 
the Ly-alpha around lambda 3600 A at a redshift z = 1.96, further 
analysis of this spectrum are ongoing. In light of this, from a refined 
analysis of our VLT/FORS2 spectrum of this GRB (D'Avanzo et al. GCN 
8345
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 ), we can identify the metal lines reported by Cucchiara et al. (GCN 
8346
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 ) and the absorption lines CrII(2056, 2062,2066) and ZnII(2062) all 
at a redshift z = 1.9685.

Our results are thus in agreement with what reported by Cucchiara et al.

GRB 081008, Swift-BAT refined analysis

D. M. Palmer (LANL), 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), C. B. Markwardt
(GSFC/UMD), K. McLean (GSFC/UMD), A. M. Parsons (GSFC), J. L. Racusin
(PSU), 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-240 to T+963 sec from the recent telemetry
downlink, we report further analysis of BAT GRB 081008 (trigger
#331093) (Racusin et al., GCN Circ. 8344
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 ).  The BAT ground-calculated
position is RA, Dec = 279.968, -57.433 deg which is
    RA(J2000)  =  18h 39m 52.4s
    Dec(J2000) = -57d 25' 58.8"
with an uncertainty of 1.1 arcmin, (radius, sys+stat, 90%
containment).  The partial coding was 81%.

The mask-weighted lightcurve shows two strong peaks at T+0 and T+110s.
T90 (15-350 keV) is 185.5 +- 40.3 sec (estimated error including
systematics).

The time-averaged spectrum from T-65 to T+201 sec is best fit by a
simple power-law model.  A cutoff-power law model is also an
acceptable fit with an E_peak of 88 keV.  The power law index of the
time-averaged spectrum is 1.69 +- 0.07.  The fluence in the 15-150 keV
band is 4.3 +- 0.2 x 10^-06 erg/cm2.  The 1-sec peak photon flux
measured from T+7.52 sec in the 15-150 keV band is 1.3 +- 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/331093/BA/

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 081008
(GCN 8344
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 , Racusin et al.).  Several dithered images were obtained
in each filter, with total summed exposure times of 180s in each
of BRIJK and 120s in each of H and V.

At a mid-exposure time of 2008-10-08 23:53 (3.9 hrs post-burst),
the GRB afterglow (GCN 8343
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 , Rykoff et al. and GCN 8344
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 , Racusin et al.)
is detected with the following magnitudes:
B = 19.86 +/- 0.07
R = 18.46 +/- 0.04
I = 18.09 +/- 0.05
J = 17.13 +/- 0.10
H = 16.52 +/- 0.08
K = 16.08 +/- 0.09

Observations were obtained under non-photometric conditions.  In optical, 
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.  In the IR, calibration is against 2MASS stars.

Additional observations were obtained at 5.0 hrs and 6.4 hrs post-burst.
Between 3.9 hrs and 6.4 hours post-burst, the afterglow decays
rapidly with an approximate optical decay rate of alpha~-1.5 (where 
afterglow flux is proportional to t^alpha).

[GCN OPS NOTE(27oct08): Per author's request, the errors for the J,H,K
measurements from 0.97,0.77,0.90 to 0.10,0.08,0.09, respectively.]

Peter Nelson (Ellinbank, Victoria, Australia) reports to the AAVSO 
International High Energy Network the following optical observation of 
GRB081008 (GCN #8344
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  Racusin et al.; GCN #8343
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  Rykoff et al.):

Peter Nelson has obtained a limiting magnitude on the optical afterglow of 
GRB081008 (Racusin et al., GCN #8344
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 ).  The optical afterglow position RA: 
18:39:50, Dec: -57:25:52 found by Rykoff et al. (GCN #8343
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 ) was observed 
for a total of 480 seconds using a 0.32-meter Newtonian with an SBIG ST8XE 
CCD and an R filter.  The mid-point of the observations was 2008 Oct 
09.4150 UT, approximately 14 hours after the burst.  The afterglow was not 
detected to an upper limit of R=18.2, using USNO-A2.0 star 0300-34687382 
(R=11.8) as the comparison.  This upper limit is consistent with the 
R-band magnitude of R=18.46 on 2008 Oct 08.9951 observed by Cobb et al. 
(GCN #8356
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 ).

A detailed report of these observations is available on the AAVSO website 
at ftp://ftp.aavso.org/grb/PeterNelson_081008_2454752.03073_.txt

A fits image is available at
ftp://ftp.aavso.org/grb/PeterNelson_081008_2454752.03073_.fits

The AAVSO thanks the Curry Foundation for their continued support of the 
AAVSO International High Energy Network.

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

"Analysis of the Gemini-South GMOS r-band acquisition image taken on 
October 8
(Cucchiara et al., 8346) shows the presence of an extended source at 
2-arcsec
distance from the afterglow of GRB 081008.  The source is serendipitously
located in the GMOS slit during our spectroscopic observations.

Extracting the spectrum of this object reveals the presence of metal 
absorption
features including FeII(1608), AlII(1670), NiII(1703), AlIII(1854), and
CrII(2056,2062,2066) at a common redshift z=1.967.  We conclude that this
galaxy is associated with the strong z=1.967 absorber observed in the GRB
afterglow spectrum, and is a candidate host galaxy to the GRB.

The galaxy is located at (J2000) RA 18:39:49.64, Dec -57:25:53.41.  At 
z=1.967
its 2.1-arcsec offset from the line of sight to GRB 081008 corresponds 
to 16 kpc
in projection.

We report the R-band magnitude of the GRB and the Host, calibrated using 
USNO
catalog magnitudes of 4 nearby USNO stars:  At the time of the 
acquisition image,
00:01 UT, we find R=18.32 +- 0.1 mag for the GRB afterglow, and R=20.75 
+- 0.1
mag for the galaxy.

The observed host galaxy magnitude corresponds to an absolute AB 
magnitude (at
central wavelength 220 nm in the host galaxy frame) of M_220(AB) = -21.5 
mag;
assuming a comparable rest-frame B-band luminosity would place this galaxy
among the brightest GRB host galaxies if it is the host galaxy of GRB 081008
(Berger et al., Apj 2007, 657).

While we cannot confirm a host identification at this time, we note that the
presence of an associated DLA in the VLT/FORS2 spectrum (d'Avanzo et al.,
GCN 8350
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 ), along with the reported absence of any absorption signature due
to neutral hydrogen at z>1.967, both support this suggestion."


-- 

Antonino Cucchiara
PhD candidate
Department of Astronomy&Astrophysics
Penn State University
website: www.astro.psu.edu/~cucchiara/

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 refined position of the GRB 081008 optical
afterglow (GCN 8348
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 ) at 4.800 and 4.928 GHz with the Australia Telescope
Compact Array (ATCA) between 01:15:05 UT, November 24, 2008 and 20:35:30
UT, November 25, 2008.

We did not detect a radio source at the optical afterglow position of
the GRB 081008 (GCN 8348
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 ). The data at 4.800 and 4.928 GHz were merged
and the radio flux density at the afterglow position found out to be
-0.151 +/- 0.156 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/grb081008_field_image