GRB 080905B

M. C. Stroh (PSU), S. D. Barthelmy (GSFC),
W. H. Baumgartner (GSFC/UMBC), A. P. Beardmore (U Leicester),
D. N. Burrows (PSU), M. M. Chester (PSU), J. R. Cummings (NASA/UMBC),
P. A. Evans (U Leicester), N. Gehrels (NASA/GSFC),
S. T. Holland (CRESST/USRA/GSFC), S. Immler (CRESST/GSFC/UMD),
J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA),
F. E. Marshall (NASA/GSFC), P. T. O'Brien (U Leicester),
C. Pagani (PSU), K. L. Page (U Leicester), B. Preger (ASDC),
J. L. Racusin (PSU), T. Sakamoto (NASA/UMBC), P. Schady (MSSL-UCL),
R. L. C. Starling (U Leicester) and L. Vetere (PSU) report on behalf
of the Swift Team:

At 16:55:45 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 080905B (trigger=323898).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 301.723, -62.571 which is 
   RA(J2000) = 20h 06m 53s
   Dec(J2000) = -62d 34' 15"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed three well separated
peaks each about 10 seconds long with a total burst duration of about 
90 sec.  The peak count rate was ~2000 counts/sec (15-350 keV), at ~4 sec 
after the trigger. 

The XRT began observing the field at 16:57:28.6 UT, 103.2 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source with an enhanced position: RA, Dec 301.74027,
-62.56347 which is equivalent to:
   RA(J2000)  = 20h 06m 57.66s
   Dec(J2000) = -62d 33' 48.5"
with an uncertainty of 2.7 arcseconds (radius, 90% containment). This
location is 39 arcseconds from the BAT onboard position, within the BAT
error circle. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density in excess of the Galactic value (3.49e+20
cm^-2, Kalberla et al. 2005), with an excess column of 1.7
(+1.47/-1.31) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 2.5 s image was 2.11e-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 112 seconds after the BAT trigger.   
The 2.7'x2.7' sub-image covers 100% of the XRT error circle.  Inside 
the XRT error circle is a 16th mag source that is starlike in the DSS,
  but appears  extended or double in the UVOT image and may be an  
optical afterglow.  A second finding chart in V does not allow us to 
 confirm or reject the candidate at this time.  No  correction has  
been made for the  expected extinction corresponding to E(B-V) of   
0.05. 

Burst Advocate for this burst is M. C. Stroh (stroh AT astroh.org). 
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. 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), D. Palmer (LANL), T. Sakamoto (GSFC/UMBC),
G. Sato (ISAS), M. Stamatikos (GSFC/ORAU), M. C. Stroh (PSU),
J. Tueller (GSFC), T. Ukwatta (GWU)
(i.e. the Swift-BAT team):
 
Using the data set from T-239 to T+903 sec from recent telemetry downlinks,
we report further analysis of BAT GRB 080905B (trigger #323898)
(Stroh, et al., GCN Circ. 8182
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 ).  The BAT ground-calculated position is
RA, Dec = 301.746, -62.568 deg, which is 
   RA(J2000)  =  20h 06m 59.1s 
   Dec(J2000) = -62d 34' 03.8" 
with an uncertainty of 1.9 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 37%.
 
The mask-weighted light curve shows two clusters pf peaks. The first starts
at ~T-2 sec and almost returns to background levels as the second cluster
starts around T+40 sec.  The second cluster returns to background at ~T+220 sec.
T90 (15-350 keV) is 128 +- 16 sec (estimated error including systematics).
 
The time-averaged spectrum from T-15.5 to T+128.5 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.78 +- 0.15.  The fluence in the 15-150 keV band is 1.8 +- 0.2 x 10^-6 erg/cm2.
The 1-sec peak photon flux measured from T+8.02 sec in the 15-150 keV band
is 0.5 +- 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/323898/BA/

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

Using 979 s of XRT Photon Counting mode data and 1 UVOT
images for GRB 080905B, 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 = 301.74122, -62.56313 which is equivalent
to:

RA (J2000): 20h 06m 57.89s
Dec (J2000): -62d 33' 47.3"

with an uncertainty of 1.7 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.

Paul M. Vreeswijk, Johan P. U. Fynbo, Daniele Malesani, Jens Hjorth 
(DARK/NBI), Antonio de Ugarte Postigo (ESO), report on behalf of a 
larger collaboration:

We observed the field of GRB 080905B (Stroh et al., GCN 8182
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 ) with the 
ESO VLT equipped with FORS2. Several spectra were obtained with 
different grism choices, beginning on 2008 Sep 6.053  UT (8.34 hr after 
the GRB), and covering the wavelength ranges 3800-9500 AA at low to 
moderate resolution.

In the acquisition images, we clearly detect a point source inside the 
refined XRT error circle (Osborne et al., GCN 8189
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 ), at the coordinates 
(J2000):

RA = 20:06:57.90
Dec = -62:33:46.8

This position is consistent with that of the UVOT candidate afterglow 
reported by Stroh et al. (GCN 8182
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 ), the object being now significantly 
fainter (R ~ 20.2). We note that this position is 3.8" off the center of 
the bright 2MASS galaxy 2MASX J20065732-6233465, also visible in the DSS 
and in the UVOT images.

Spectroscopy of the afterglow reveals resonance absorption features from 
several ions, including OI, SiII, CIV, FeII, AlII, AlIII, MgII, MgI, at 
a redshift z = 2.374. The detection of transitions from fine-structure 
levels of FeII indicates that this is the redshift of the GRB, under the 
assumption that these levels are populated by GRB afterglow ultraviolet 
photons. We also find evidence for an intervening absorber at z = 1.438.

We acknowledge excellent support from the Paranal staff, in particular 
Elena Mason and Andres Pino.

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

       The Swift/UVOT observed the field of GRB 080905B starting 94 s
after the BAT trigger (Stroh et al., 2008, GCNC. 8182).  Settled
observations started at T + 113 s.  We detect the optical afterglow
(Stroh et al., GCNC 8182) in the v and white filters at the
UVOT-enhanced location of the XRT afterglow (Osborne, et al., 2008,
GCNC 8089).  There is an uncertain detection in the b filter.  The
refined UVOT source position is

  RA (J2000.0) =  20:06:57.89
Dec (J2000.0) = -62:33:47.0

with an estimated uncertainty of 0.7 arcsec (radius, 90%
confidence).  We note that the afterglow is slightly blended with the
USNO-B1.0 star 274-094527, which makes it dificult to confirm the
detection in the b band.

     Magnitudes and upper limits are reported below.

Filter   TSTART   TSTOP     Exposure     Mag   Err  Comment
  white      113     213           98   18.70  0.09
    v        219     619          394   16.60  0.07
    b        699    6033          206   19.49  0.15  uncertain detection
    u        674    5828          216  >20.7         3-sigma upper limit
   uvw1      650    5623          197  >20.3         3-sigma upper limit
   uvm2     5218    5418          197  >19.9         3-sigma upper limit
   uvw2      729    6445          197  >20.5         3-sigma upper limit

The above magnitudes are not corrected for the Galactic extinction
corresponding to a reddening of E_{B-V} = 0.05 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 possible
detection in the b filter, coupled with the non-detections in the u,
uvw1, uvm2, and uvw2 filters, are consistent with a redshift of
approximately z < 3.3.  The power-law decay index in the v band
between 763 and 6505 s is -0.88.

M.C. Stroh (PSU) reports on behalf of the Swift-XRT team:

We have analyzed the Swift-XRT data obtained for GRB 080905B (Stroh et  
al., GCN Circ. 8182
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 ), totaling 85s of Windowed Timing and 10.2ks of  
Photon Counting data.

The 0.3-10 keV X-ray light-curve can be fit by a double broken power- 
law with an initial decay index of 2.46 +/- 0.18, second decay index  
of 0.01 +/- 0.18, and a third decay index of 1.41 +/- 0.4 with first  
and second break points at 258s and 2734s respectively. We predict  
counting rates of 7.88e-3 cts/s and 4.44e-3 cts/s at T+48 and T+72  
hours.

The spectrum from the Windowed Timing data can be fit with a power-law  
in excess of the Galactic value (3.5e20 cm-2, Kalberla et al. 2005),  
with an intrinsic column density of 1.32 (+0.57/-0.48)e22 cm^-2 and a  
photon index of 1.49 (+0.13/-0.12). The observed (unabsorbed) 0.3-10  
keV flux is 5.34e-10 (6.1e-10) ergs/cm^2/s.

The spectrum from the Photon Counting data can be fit with a power law  
with intrinsic column density of 2.26 (+0.53/-0.47)e22 cm^-2 and a  
photon index of 1.92 (+0.10/-0.10). The observed (unabsorbed) 0.3-10  
keV flux is 1.259e-10 (1.7e-10) ergs/cm^2/s.

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

P. N. Bhat (UAH), W. Paciesas (UAH) and A.J. van der Horst (NASA/ORAU)
report on behalf of the Fermi Gamma-ray Burst Monitor Team:


"At 16:55:46 UT on 5 September 2008, the Fermi GBM triggered
and located GRB 080905B (trigger 242326547 / 080905705), which was
also detected by Swift (Stroh et al., GCN 8182
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 ). The on-ground calculated
location, using the Fermi GBM trigger data, is RA = 307.1, Dec = -60.2
(J2000 degrees) (equivalent to J2000 20h 28m, -60d 12'), with a statistical
uncertainty of 5.7 degree (radius, 1-sigma containment; there is
additionally a systematic error which is currently estimated to be 2 to 3
degrees). The angle from the Fermi LAT boresight to the GBM localization
is 82 degrees. 

This long GRB consists of one main peak at trigger time, possibly a weak
pre-trigger at T-20 s and second peak at T0 + 70 sec and the third from
T0+80 sec to T0+110 sec similar to the Swift light curve.

The burst duration (in the energy range 20-1000 keV) T90 is about 159 sec.

The time-averaged spectrum from T0-20 to T0+110 sec is well fit by a power
law function with index -1.75 +/- 0.12.The fluence (20-1000 keV) is
4.1(+/-0.3)E-08 erg/cm2and the peak flux (20-1000 keV) is 0.21 +/- 0.02
ph/cm2/s.

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

John French (UCD School of Physics, DARK/NBI), Gary Melady (UCD School of Physics), Daniele Malesani, Paul Vreeswijk (DARK/NBI) report on behalf of
a larger collaboration:

The Watcher 40 cm robotic telescope at Boyden Observatory, South Africa, began imaging the field of GRB 080905B (Stroh et al al. GCN 8182
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 ) at
16:56:28 UT, 43 s after the Swift triggger. In a stacked sequence of 10 unfiltered 10 s exposures, with a mid-time of 16:57:37 UT (112 s after the
Swift trigger), we detect a point source inside the refined XRT error circle (Osborne et al. GCN 8189
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 ), at a position consistent with the
detections reported by Holland et al. (GCN 8192
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 ) , Vreeswijk et al. (GCN 8191
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 ) and Stroh et al. (GCN 8182
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 ).

We measure a magnitude of 16.2 +/- 0.2 for the source, based on comparison with the USNO-B1 star at RA 20:06:51.361, Dec -62:32:52.55. Further
analysis is ongoing.

Stephen Holland (CRESST/USRA/GSFC) reports, on behalf of the
Swift/UVOT team:

      We have performed UVOT photometry on the 2MASS galaxy 2MASX
J20065732-6233465 galaxy located 4 arcsec from the optical afterglow
of GRB 080905B (Stroh et al., 2008, GCNC 8182).  Our magnitudes are:

Filter    Mag  Err
    v    17.85 0.05
    b    18.70 0.05
    u    18.46 0.05
  uvw1   18.76 0.07
  uvm2   18.71 0.07
  uvw2   18.76 0.07

These magnitudes are not corrected for the Galactic extinction
corresponding to a reddening of E_{B-V} = 0.05 mag (Schlegel et al.,
1998, ApJS, 500, 525) along this line of sight.  The photometry is on
the UVOT photometric system described in Poole et al. (2008, MNRAS,
383,627).

      The detection in the uvw2 filter implies that the redshift of the
2MASS galaxy can not be greater than approximately z = 1.7.    This is
inconsistent with the VLT redshift of z = 2.374 for the afterglow
(Vreeswijk, et al., 2008, GCNC 8191).  This suggests that the 2MASS
galaxy is not the host galaxy of GRB 080905B.