GRB 080903

D. Grupe (PSU), W. H. Baumgartner (GSFC/UMBC), D. N. Burrows (PSU),
M. M. Chester (PSU), J. R. Cummings (NASA/UMBC),
S. T. Holland (CRESST/USRA/GSFC), J. A. Kennea (PSU),
C. B. Markwardt (CRESST/GSFC/UMD), F. E. Marshall (NASA/GSFC),
D. M. Palmer (LANL), T. N. Ukwatta (GSFC/GWU) and L. Vetere (PSU)
report on behalf of the Swift Team:

At 01:12:23 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 080903 (trigger=323542).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 86.797, +51.326 which is 
   RA(J2000) = 05h 47m 11s
   Dec(J2000) = +51d 19' 35"
with an uncertainty of 3 arcmin (radius, 90% containment, including
systematic uncertainty).  The BAT light curve showed a broad single-peaked
structure with a duration of about 50 sec.  The peak count rate
was 1250 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 01:13:21.8 UT, 58.5 seconds after
the BAT trigger. Using promptly downlinked data we find a bright, fading,
uncatalogued X-ray source located at RA, Dec 86.79071, 51.26363 which
is equivalent to:
   RA(J2000)  = 05h 47m 9.77s
   Dec(J2000) = +51d 15' 49.1"
with an uncertainty of 3.0 arcseconds (radius, 90% containment). This
location is 224 arcseconds from the BAT onboard position, outside the
on-board BAT error circle, but within the BAT ground-calculated error
circle. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of
1.67e+21 cm^-2 (Kalberla et al. 2005). 

The initial flux in the 2.5 s image was 1.80e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 400 seconds with the V filter
starting 75  seconds after the BAT trigger. No afterglow candidate
has been found in the  initial data products. The 2.7'x2.7' sub-image
covers 100% of the XRT error  circle. The 3-sigma upper limit at the
location of the XRT candidate is 19.4 mag. No correction has  been
made for the expected extinction of about 0.7 magnitudes. 

Burst Advocate for this burst is D. Grupe (grupe 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.)

F. Yuan (U Mich), W. Rujopakarn (Steward), H. Swan (U Mich), D.A.  
Smith (Guilford), T. Guver (U Arizona) report on behalf of the ROTSE  
collaboration:

ROTSE-IIId, located at the Turkish National Observatory at  
Bakirlitepe, Turkey, responded to GRB 080903 (Swift trigger 323542;  
Grupe et al., GCN 8169
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 ), producing images beginning 6.7 s after the  
GCN notice time. An automated response took the first image at  
01:12:47.7 UT, 24.4 s after the burst, and during the gamma-ray  
emission, under fair conditions. We took 10 5-sec, 10 20-sec and 30 60- 
sec exposures. These unfiltered images are calibrated relative to USNO  
A2.0 (R). Imaging is on going.

Comparison to the DSS (second epoch) reveals no new sources within the  
3-sigma Swift/BAT error circle or the XRT error circle, for both  
single images and coadding into sets of 10. Individual images have  
limiting magnitudes ranging from 16.5-17.7; we set the following  
specific limits.

start UT       end UT      t_exp(s)   mlim   t_start-tGRB(s)  Coadd?
--------------------------------------------------------------------
01:12:47.7   01:14:55.9       128     17.7           24.4       Y
01:15:04.6   01:19:42.4       277     18.3          161.3       Y

Klotz, A. (CESR-OMP), Boer M. (OHP), Atteia J.L. (LATT-OMP) report:

We imaged the field of GRB 080903 detected by SWIFT
(trigger 323542) with the TAROT robotic telescope (D=25cm)
located at the Calern observatory, France.

The observations started 29.1s after the GRB trigger
(11.4s after the notice). The elevation of the field increased
from 38 degrees above horizon and weather conditions
were excellents.

The date of trigger : t0 = 2008-09-03T01:12:23.3

The first image is trailed with a duration of 60.0s
(see the description in Klotz et al., 2006, A&A 451, L39).
We do not detect any OT at the XRT position provided
by Grupe et al. (GCNC 8169):
t0+29.1s to t0+89.1s : R > 17.1

The second image is 30.0s exposure in tracking mode:
t0+95.9s to t0+125.9s : R > 18.2

Magnitudes were estimated with the nearby USNO-B1 stars
and are not corrected for galactic dust extinction.

N.B. Galactic coordinates are lon=160.7489 lat=+11.6608
and the galactic extinction in R band is 0.6 magnitudes
estimated from D. Schlegel et al. 1998ApJ...500..525S.

This message may be cited.

A. de Ugarte Postigo (ESO, Chile), A.J. Castro-Tirado,
V. Casanova, J. Gorosabel, M. Jelinek, M. Guerrero
(IAA-CSIC, Granada) report on behalf of a larger collaboration:

We have observed the field of GRB 080903 (Grupe et al., GCN 8169
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 )
using the 1.5m OSN telescope at Sierra Nevada Observatory (Granada)
in V, R and I bands begining at 02:30 UT (78 min after the burst).
We detect a source of I~21.0 on the edge of the XRT error box at
the following coordinates (J2000 +/- 0.5"):

R.A.: 05:47:10.08
Dec.: +51:15:50.9

Further observations are ongoing from OSN and NOT to check the
colour and variability of this source.

S. Korotkiy (Ka-Dar Observatory), T. Kryachko, B. Satovskiy (Astrotel 
Observatory, KSU/AST) and D. Denisenko (IKI) report:

We have observed the error box of Swift gamma-ray burst GRB 080903 
(Grupe et al., GCN 8169
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 ) with 0.3-m f/7.8 Astrotel-Caucasus telescope (+ 
Apogee Alta U9000 CCD camera) at Kazan State University Mountain Station 
(Karachay-Cherkessia, Russia).  The first 120-sec unfiltered exposure 
was taken at 01:18:21 UT, 358 seconds after the burst trigger.  Inside 
the XRT error circle we have found a new source not present on DSS 
plates at the following coordinates (J2000.0):

05 47 10.05 +51 15 50.9

(astrometry relative to USNO-A2.0 catalogue, accuracy +/-0.2").

Using the nearby USNO-A2.0 1350-06155883 (05 47 16.36 +51 15 56.4) with 
R=18.1 as a comparison star, we have measured CR (unfiltered with Red 
zero point) magnitude 19.1 for the afterglow candidate.  The limiting 
magnitude of the image is 20.5.

Four additional images were obtained with the approaching twilight and 
brightening sky background.  The object is still present on the second 
120-sec exposure obtained at 01:22:09 UT, but not detected on the next 
images.

The crop of Astrotel-Caucasus image is available at:

http://www.astroalert.su/files/grb080903_crop_x4_size.jpg

Color-combined (BRIR) DSS image of the same area for comparison:

http://hea.iki.rssi.ru/~denis/GRB080903-BRIR-crop.jpg

C. Guidorzi (INAF-OAB), R. J. Smith (LJMU), A. Gomboc (U. Ljubljana)
on behalf of a larger collaboration report:

On 2008 September 03 (03:32:18 UT) we began observing the field of
GRB 080903 (Grupe et al. GCN Circ. 8169
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 ) with the Liverpool Telescope
using the SDSS i filter.

We clearly detect the optical afterglow candidate reported by de
Ugarte Postigo et al. (GCN Circ. 8172
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 ) and Korotkiy et al.
(GCN Circ. 8173
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 ). We found the following magnitudes:

Telescope   Filter  T_mid[hr]   Exposure[s]      Mag
-------------------------------------------------------------
LT          i'         2.59       6x300        21.32 +/- 0.07
LT          i'         4.08       6x300        21.52 +/- 0.11
-------------------------------------------------------------

The calibration was performed using the I magnitude of the following
nearby USNOB1 catalogue star: 05:47:14.30, +51:15:32.8, using I=14.77.

Andreev M., Sergeev A., Karpov N. (Terskol Branch of Institute of
Astronomy),  Khramtsova M. (The Ural State University),  Petkov V.(BNO INR
RAS), A. Pozanenko (IKI) on behalf of larger GRB follow up collaboration
report:

We observed the field of GRB 080903 (Grupe et al. GCN 8169
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 )  with Z-600 and
Celestron 11" telescopes of Mt. Terskol observatory.  A set of  60 s
exposures in R filter was taken with Z-600 starting 22 minutes after burst
onset. In a combined image we clearly detect afterglow candidate (Ugarte
Postigo et al. GCN 8172
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 , Korotkiy et al. GCN 8173
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 ). Preliminary photometry
is based on USNO-A2.0 1350-06155883 star (05 47 16.36 +51 15 56.4) assuming
R=18.1:

T0+       Exp.        R_mag
(Mid)

1673 sec. 10x60 sec.  19.35 +/-0.2

The afterglow is also detected in unfiltered combined image (40x10s) taken
with Celestron 11" telescope at Tmid = 1280 s. The photometry is underway.

J. Tueller (GSFC), S. D. Barthelmy (GSFC), W. Baumgartner (GSFC/UMBC),
J. Cummings (GSFC/UMBC), E. Fenimore (LANL), N. Gehrels (GSFC),
D. Grupe (PSU), H. Krimm (GSFC/USRA), C. Markwardt (GSFC/UMD),
K. McLean (GSFC/UMD), D. Palmer (LANL), T. Sakamoto (GSFC/UMBC),
G. Sato (ISAS), M. Stamatikos (GSFC/ORAU), T. Ukwatta (GWU)
(i.e. the Swift-BAT team):
 
Using the data set from T-239 to T+723 sec from recent telemetry downlinks,
we report further analysis of BAT GRB 080903 (trigger #323542)
(Grupe, et al., GCN Circ. 8169
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 ).  The BAT ground-calculated position is
RA, Dec = 86.790, 51.256 deg, which is 
   RA(J2000)  =  05h 47m 09.6s 
   Dec(J2000) = +51d 15' 22.4" 
with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 100%.
 
The mask-weighted light curve shows a single FRED peak starting at ~T-5 sec
and ending at ~T+80 sec.  There appears to be very low level emission out
to ~T+500 sec.  T90 (15-350 keV) is 66 +- 11 sec (estimated error
including systematics).
 
The time-averaged spectrum from T-10.5 to T+68.0 sec is best fit by a power law
with an exponential cutoff.  This fit gives a photon index 0.84 +- 0.52, 
and Epeak of 60 +- 15 keV (chi squared 63 for 56 d.o.f.).  For this
model the total fluence in the 15-150 keV band is 1.4 +- 0.1 x 10^-6 erg/cm2
and the 1-sec peak flux measured from T-2.46 sec in the 15-150 keV band is
0.8 +- 0.1 ph/cm2/sec.  A fit to a simple power law gives a photon index
of 1.72 +- 0.10 (chi squared 73 for 57 d.o.f.).  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/323542/BA/

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

Using 3613 s of XRT Photon Counting mode data and 3 UVOT
images for GRB 080903, 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 = 86.79206, +51.26433 which is equivalent
to:

RA (J2000): 05h 47m 10.10s
Dec (J2000): +51d 15' 51.6"

with an uncertainty of 1.6 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.

Dirk Grupe (PSU) and Phil Evans (U Leicester) report on behalf of the 
Swift-XRT team:


The XRT began observing the field of GRB 080903 (trigger=323542; Grupe 
et al.,
GCN Circ 8169
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 ) at 01:13:21.8 UT, 58.5 seconds after
the BAT trigger.

The enhanced XRT position given in GCN Circ 8177
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  (Goad et al)
is 30.3" away from the ground-processed BAT position given in GCN Circ. 
8176
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(Tueller et al.),  consistent with that position.

The 0.3-10 keV X-ray light curve starts at a level of 2e-9 ergs/s/cm2 (40
counts/s) in the XRT with a rather flat decay slope at these early times of
alpha = 1.00+/-0.04. The light curve breaks at about 1 ks after the 
burst and
steepens dramatically to alpha = 2.47+/-0.14. If this decay slope 
continues as is,
we predict an XRT count rate at T0+24 hours of 3e-5 counts/s or 2e-15
ergs/s/cm2, so undetectable for XRT.


The spectrum of the Windowed Timing data of the first orbit
can be fitted by an absorbed single powerlaw with an unusual flat photon
index Gamma = 0.94 +/- 0.06 and column density fixed to the Galactic 
value of
1.67e21 cm-2 in the direction of the burst. This photon index is 
consitent with
that found by the BAT in the 15-150 keV band (Gamma=0.84+/-0.52; Tueller 
et al.,
GCN Circ. 8176
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 ). The spectrum becomes slightly steeper at later times.
The Photon Counting mode data show an photon spectral slope of 
Gamma=1.23+/-0.16
with the absorption column density fixed to the Galactic value.


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

F.E. Marshall (NASA/GSFC) and D. Grupe (PSU)
report on behalf of the Swift/UVOT team:

The Swift/UVOT observed the field of GRB 080903 starting 75 s after
the BAT trigger (Grupe et al., GCN Circ. 8169
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 ). A source is
detected in the initial exposure in the V filter at the position
of the optical afterglow (de Ugarte Postigo et al., GCN Circ. 8172
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 ).
There is also a marginal (1.8 sigma) detection in the initial exposures
with the U filter. A valid detection in the U filter would
indicate a redshift of less than 3.3.

The magnitudes and 3-sigma upper limits in the UVOT photometric system 
(Poole et al. 2008, MNRAS, 383, 627) for detecting
a source in the first finding chart (FC) exposure and 
subsequent co-added exposures are:

Filter          T_start(s)   T_stop(s)      Exp(s)       Mag 

v (FC)              75          475          393         19.8 +/- 0.24
v                  595        28705         2596        >21.5
b                  555        12429         1307        >21.8
u                  530          829           58         20.0 +/- 0.47
u                 4906        34528         3049        >21.8
uvw1               506         7268          551        >21.0
uvm2               481         7160          655        >20.7
uvw2               571         6750          452        >21.2

The values quoted above are not corrected for the Galactic extinction due
to the reddening of E(B-V) = 0.22 in the direction of the burst 
(Schlegel et al. 1998).