GRB 060908

P. A. Evans (U Leicester), S. D. Barthelmy (GSFC),
A. P. Beardmore (U Leicester), D. N. Burrows (PSU),
N. Gehrels (NASA/GSFC), O. Godet (U Leicester),
C. Guidorzi (Univ Bicocca&INAF-OAB), J. A. Kennea (PSU),
A. N. Morgan (PSU), K. L. Page (U Leicester), D. M. Palmer (LANL),
P. Romano (INAF-OAB), G. Sato (GSFC/ISAS), M. Stamatikos (NASA/ORAU)
and H. Ziaeepour (UCL-MSSL) report on behalf of the Swift Team:

At 08:57:22 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 060908 (trigger=228581).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA,Dec 31.835, +0.370 {02h 07m 20s, +00d 22' 14"} (J2000)
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows 3 overlapping peaks
starting at ~T-10 sec and then a 4th peak lasting until T+12 sec. 
The peak count rate was ~3000 counts/sec (15-350 keV), at ~T+2 sec
after the trigger. 

The XRT began observing the field at 08:58:34 UT, 72 seconds after the
BAT trigger. XRT found a fading and uncatalogued X-ray source
located at RA(J2000) = 02h 07m 18.1s, Dec(J2000) = +00d 20' 29.1", with an
estimated uncertainty of 3.8 arcseconds (90% confidence radius). 
This location is 108 arcseconds from the BAT on-board position, within
the BAT error circle. The initial flux in the 2.5s image was
5.0e-10 erg/cm2/s (0.2-10 keV). 

UVOT took a finding chart exposure of 100 seconds with the White
(160-650 nm) filter starting 80 seconds after the BAT trigger. There
is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image
at (RA,DEC) (J2000) of (31.8265,0.3420) or (02h07m18.36s,+00o20'31.2")
 with a 1-sigma error radius of about 0.7 arc sec. This position is
3.8 arc sec. from the center of the XRT error circle. The estimated
magnitude is 15.1 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 0.03.

M. Nysewander, D. Reichart, K. Ivarsen, A. Foster, A. LaCluyze, J.A. Crain,
report:

SkyNet observed the localization of GRB 060908 with four of the PROMPT
telescopes starting at 08:58:25 UT.

We note a very bright uncatalogued rapidly fading source at approximately
02:07:19, +00:20:30.

In the r' initial image beginning at 08:59:07 UT, 105 s after the trigger,
the source is at approximately r ~ 15.

Further analysis is underway.

L.A. Antonelli, S. Covino, V. Testa,  E. Palazzi, E. Molinari, P. 
D'Avanzo, G. Chincarini, F.M. Zerbi, G. Tosti, F. Vitali, P. Conconi, 
G. Cutispoto, G. Malaspina, L. Nicastro, E. Meurs, and P. Goldoni 
report on behalf of the REM/ROSS team:

The robotic 60-cm REM telescope, located at La Silla (Chile), observed 
automatically the field of GRB 060908 (Evans et al., GCN 5544
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 ) with 
ROSS optical camera. Observations started at 09:06:21 UT, about 7 min 
after the burst,  in R, I and V band.

An uncatalogued fading source was observed in all filters at:

RA: 02:07:18.3
DEC +00:20:31

This position is consistent with the one reported in GCN 5544
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  and GCN 
5545
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 .

In the R initial image the source is at approximately R ~ 17 in 20 sec 
exposure time.

Further analysis is in progress.

Richard J. Cool (Arizona), Daniel J. Eisenstein (Arizona), David
W. Hogg (NYU), Michael R. Blanton (NYU), David J. Schlegel (LBNL),
J. Brinkmann (APO), Donald Q. Lamb (Chicago), Donald P. Schneider
(PSU), and Daniel E. Vanden Berk (PSU) report:

The Sloan Digital Sky Survey (SDSS) imaged the field of burst
GRB060908 prior to the burst.  As these data should be useful
as a pre-burst comparison and for calibrating photometry, we are
supplying the images and photometry measurements for this GRB field
to the community.

Data from the SDSS, including 5 FITS images, 3 JPGS, and
3 files of photometry and astrometry, are being placed at
http://mizar.as.arizona.edu/~grb/public/GRB060908

We supply FITS images in each of the 5 SDSS bands of a 8'x8'
region centered on the GRB position (ra=31.8350 (02:07:20.4),
dec=0.370000 (00:22:12.0); Swift-BAT TRIGGER 228581), as well as 3
gri color-composite JPGs (with different stretches). The units in
the FITS images are nanomaggies per pixel.  A pixel is 0.396 arcsec
on a side. A nanomaggie is a flux-density unit equal to 10^-9 of
a magnitude 0 source or, to the extent that SDSS is an AB system,
3.631e-6 Jy.  The FITS images have WCS astrometric information.

In the file GRB060908_sdss.calstar.dat, we report photometry
and astrometry of 37 bright stars (r<20.5) within 15' of the
burst location.  The magnitudes presented in this file are asinh
magnitudes as are standard in the SDSS (Lupton 1999, AJ, 118,
1406). Beware that some of these stars are not well-detected in
the u-band; use the errors and object flags to monitor data quality.

In the files GRB060908_sdss.objects_flux.dat and
GRB060908_sdss.objects_magnitudes.dat, we report photometry
of 1285 objects detected within 6' of the GRB position.
We have removed saturated objects and objects with model
magnitudes fainter than 23.0 in the r-band.  The fluxes listed
in GRB060908_sdss.objects_flux.dat are in nanomaggies while the
magnitudes listed in GRB060908_sdss.objects_magnitudes.dat are
asinh magnitudes.

All quantities reported are standard SDSS photometry, meaning that
they are very close to AB zeropoints and magnitudes are quoted
in asinh magnitudes.  Photometric zeropoints are known to about
2% rms.  None of the photometry is corrected for dust extinction.
The Schlegel, Finkbeiner, and Davis (1998) predictions for this
region are A_U=0.146 mag, A_g=0.107 mag, A_r = 0.078 mag, A_i=0.059
mag, and A_z=0.042 mag.

The file GRB060908_sdss.spectro.dat contains a list of the 11 objects
with SDSS spectroscopy within 6 arcminutes of the GRB position.
In addition to the redshift and 1-sigma error for each object,
this file also lists the object spectroscopic classification.


SDSS astrometry is generally better than 0.1 arcsecond per
coordinate.  Users requiring high precision astrometry should take
note that the SDSS astrometric system can differ from other systems
such as those used in other notices; we have not checked the offsets
in this region.

More detailed information pertaining to our SDSS GRB releases
can be found in our initial data release paper (Cool et al. 2006,
astro-ph/0601218).  See the SDSS DR4 documentation for more details:
http://www.sdss.org/dr5.

These data have been reduced using a slightly different pipeline
than that used for SDSS public data releases.  We cannot guarantee
that the values here will exactly match those in the data release
in which these data are included.  In particular, we expect the
photometric calibrations to differ by of order 0.01 mag.

This note may be cited, but please also cite the SDSS data release
paper, Adelman-McCarthy et al. (2006, ApJS, 162, 38), when using
the data or referring to the technical documentation.

D. Palmer (LANL), L. Barbier (GSFC), S. D. Barthelmy (GSFC),
J. Cummings (GSFC/UMD), P. A. Evans (U Leicester), E. Fenimore (LANL),
N. Gehrels (GSFC), D. Hullinger (BYU-Idaho), H. Krimm (GSFC/USRA),
C. Markwardt (GSFC/UMD), A. Parsons (GSFC), T. Sakamoto (GSFC/ORAU),
G. Sato (GSFC/ISAS), M. Stamatikos (GSFC/ORAU), J. Tueller (GSFC)
on behalf of the Swift-BAT team:
 
Using the data set from T-240 to T+302 sec from recent telemetry downlinks,
we report further analysis of BAT GRB 060908 (trigger #228581)
(Evans, et al., GCN Circ. 5544
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 ).  The BAT ground-calculated position is
RA,Dec = 31.820, 0.330 deg {2h 7m 16.8s,+0d 19' 48.9"} (J2000) +- 1.5 arcmin,
(radius, sys+stat, 90% containment).  The partial coding was 46%.
 
The lighcurve has 3 overlapping peaks starting at ~T-12 sec then briefly
returning to background at T+6 sec.  The second peak has a maximium
at T+10 sec and ends at T+14 sec.  T90 (15-350 keV) is 19.3 +- 0.3 sec
(estimated error including systematics).
 
The time-averaged spectrum from T-17.5 to T+15.1 is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.33 +- 0.07.  The fluence in the 15-150 keV band is 2.9 +- 0.1 x 10^-6 erg/cm2.
The 1-sec peak photon flux measured from T+0.97 sec in the 15-150 keV band
is 3.2 +- 0.2 ph/cm2/sec.  All the quoted errors are at the 90% confidence level.

K. Wiersema (University of Amsterdam), C.C. Thoene (DARK Cosmology Centre)
and E. Rol (University of Leicester) report:

We observed the position of GRB 060908 using the Danish 1.5m telescope
equipped with DFOSC, in R band. The optical transient (Evans et al,
GCN 5544
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 ) is detected in all our exposures.

We performed photometry with respect to a nearby USNO-B1.0 star
0903-0022606, for which we assume R = 18.29.  The reported error in
our magnitude is therefore only the error in the photometry of the
transient. We find R = 21.57 +/- 0.04, 46 minutes after the trigger.

During our observations between 36 and 70 minutes after the trigger,
the optical transient is clearly decaying: a power law fit to our data
results in a decay index of 1.07 +/- 0.11. The magnitude and decay
rate suggest that the decay slowed down somewhat, after an initial
steep decline.

A. N. Morgan (PSU),  D. E. Vanden Berk (PSU), P. Brown (PSU) and P. A.
Evans (U Leicester) report on behalf of the Swift UVOT team:

The Swift/UVOT began observations of GRB 060908 at 08:58:42 UT, ~80
seconds after the initial Swift BAT trigger (Evans et al., GCN Circ.
5544
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 ). An optical counterpart was detected with the V filter at a
position RA(J2000) = (02h 07m 18.36s,+00d 20' 31.2") to within 0.7".

An optical afterglow was detected in the White, V, B, U, and UVW1 filters.

The best estimate for the photometric redshift is z=1.62 assuming an
SMC extinction law with E(B-V)=0.05, but it is poorly constrained.
Values range from z=0.26-2.20 for a 90% confidence range.

The photometry was performed using a 6 arcsec radius aperture and is
reported below:

Filter  T_range(s)    Exp. (s)   Magnitude
White   80-179        99.8       15.06 +/- 0.03
White   5839-6038     199.8      19.57 +/- 0.22

V       185-584       399.8      16.85 +/- 0.05
V       4816-6448     399.6      19.44 +/- 0.36

B       663-820       19.6       18.41 +/- 0.31
B       5635-7129     262.5      > 20.03 (3.0-sigma)

U       639-658       19.7       17.01 +/- 0.16
U       786-805       19.8       17.99 +/- 0.32

UVW1    615-782       39.6       18.61 +/- 0.35
UVW1    5226-6857     399.6      > 20.24 (3.0-sigma)

UVM2    591-758       39.6       > 18.57 (3.0-sigma)
UVM2    5021-6652     399.6      > 20.54 (3.0-sigma)

UVW2    691-858       39.6       > 19.07 (3.0-sigma)
UVW2    4612-6243     399.6      > 20.71 (3.0-sigma)

The values quoted above are not corrected for the expected Galactic
extinction of E(B-V)=0.03.

P.A. Evans, A.P. Beardmore, O. Godet and K.L. Page (U. Leicester) report on
behalf of the Swift/XRT team:

We have analysed the first 2 orbits of Swift data of GRB 060908 (Evans et al.,
GCN 5544
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 ), totalling 100 s of data in Window Timing mode (WT) and 2.5 ks in 
Photon Counting mode (PC).

The PC mode image provides a refined XRT position of

RA(J2000)   =  02 07 18.21
Dec(J2000)  = +00 20 28.8

with an error of 3.6" (90% confidence). This position is within 1.5" of the
initial XRT position, and 3.4" from the UVOT position, reported in GCN 5554
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 .

Spectral fits to the first orbit of data suggest an absorption column in excess
of the Galactic value (2.4e20 cm^-2). The WT data yielded 8.0+4.9/-4.0 e20 cm^-2
and the PC mode data gave 6.4+3.4/-3.0 e20 cm^-2.

There is also evidence for spectral evolution during the first orbit: the WT
mode photon index was 2.43 (+0.30, -0.26), while the PC mode photon index was
1.95 (+0.15,-0.14). The observed PC mode flux was 7.5e-11 erg/s/cm^2 during the
first orbit; which translates to an unabsorbed flux of 8.9e-11 erg/s/cm^2

The lightcurve of the first 2 orbits (T0+78 to T0+6600) can be modelled with
a single power-law, and a flare at around 800s. The decay slope is 1.11
+/-0.06, and we thus predict a count rate of 0.004 counts per second 24 hours
after the trigger. This translates to a flux of 1.56e-13 erg/s/cm^2, or an
unabsorbed flux of 1.85e-13 erg/s/cm^2.

This circular is an official product of the Swift XRT Team.

-- 
-------------------------

Phil Evans,
PDRA/Swift Scientist
X-ray and Observational Astronomy Group,
University of Leicester

Tel: (0116) 252 5059
pae9@star.le.ac.uk
http://www.astro.keele.ac.uk/~pae

E. Rol (U of Leicester), P. Jakobsson (U of Hertfordshire), N. Tanvir
(U of Leicester), A. Levan (U of Hertfordshire), report for a larger
collaboration:

We obtained a spectrum of GRB 060908 (Evans et al, GCN 5544
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 ) with 
Gemini-North/GMOS, for a total of 30 minutes starting approximately 2 
hours post burst. We identify absorption lines of CIV (1548, 1551) and
SiII (1304), and possibly AlIII (1863), at a redshift of z = 2.43 +/- 0.05.

We thank the Gemini staff for their support with the observations.

P. Chandra (UVA/NRAO) and  D. A. Frail (NRAO) report on behalf of
the Caltech-NRAO-Carnegie GRB Collaboration:

"We observed the field centered on the BAT position of the Swift burst GRB
060908 (GCN#5544
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 ) using the VLA at a frequency of 8.46 GHz and starting at
7.55 UT on Sept 1st, a day after the burst. There is no detection of the
GRB with 2-sigma upper limit of 51 microJy.

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

M. Andreev, A. Sergeev (Terskol Branch of Institute of Astronomy), A.
Kurenya (Baksan Neutrino Observatory), A. Pozanenko (IKI) on behalf of
larger GRB follow up collaboration report:

We observed the afterglow of GRB060908  (Evans et al., GCN 5544
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 ) with 2m
telescope of Terskol Observatory on Sep. 08 (UT) 20:51:45 - 21:14:45. We
detect the afterglow on a combined image of 20x60 s exposure. Based on USNO
A2.0 field stars we estimate brightness of the optical transient:

Mid time (UT), Exposure, R_mag

Sep. 08.877    20x60 s   20.70 +/- 0.27

It should be noted that R magnitude of the star  (USNO-B1.0 0903-0022606  R
= 18.29) used in GCN 5552
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  (Wiersema et. al) is inconsistent with the
R-magnitude of the same star USNO A2.0 (0900-00494220   R = 14.9).
R-magnitudes of other field stars of the two catalogs USNO-B1.0 and USNO
A2.0 are also inconsistent by ~2.9 mag in average.

The message may be cited.

Christina C. Thoene (DARK Cosmology Centre), Christina Henriksen (DARK,
NOT) and Klaas Wiersema (Univ. of Amsterdam) report:

We observed the field of GRB060908 on Sep. 21 and 23 with ALFOSC at the
NOT on La Palma in V, R and i under variable conditions.
Near the position of the afterglow (GCN 5544
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 ), we detect a faint, extended
source in stacked images in V (7x600s) and R (10x600s), which we assume to
be the host galaxy of GRB060908. The possible host is not visible in
7x600s stacked images in the i band which might be due to severe fringing
in the i band images.
Photometry of the source gives values of V=24.9 +-0.1, R=25.12 +-0.05 and
i>24.6 +-0.2 using photometric zeropoints from the NOT/ALFOSC webpage.


An image of the host in the R-band can be found at
www.astro.ku.dk/~cthoene/GRBs/