GRB 080805

P. Jakobsson (U. Hertfordshire), J. P. U. Fynbo, P. M. Vreeswijk
(DARK/NBI) and A. de Ugarte Postigo (ESO) report on behalf of a
larger collaboration:

We have reduced the VLT spectra presented in Jakobsson et al.
(GCN 8062
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 ) with new calibration files and performed a thorough
line search in all four grisms (300V, 1028z, 1200R, 1400V)
covering approximately 3700-9500 A. We confirm the existence
of a line system at redshift z = 1.505 +/- 0.001 displaying
Fe II (2382,2586,2600), the Mg II doublet (2796,2803) and
Mg I (2852). In addition, there is a clear bump (flux decrease)
present between 5300-5600 A that can be interpreted as the
2175 A extinction feature at z ~ 1.5.

Apart from the z = 1.20 redshift system presented in GCN 8062
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 ,
no other significant lines are visible. We therefore conclude
that z = 1.505 is the redshift of GRB 080805.

C. Guidorzi (INAF-OAB), I. Steele and R.J. Smith (LJMU) on behalf of a
larger collaboration report:

On 2008 August 05 (10:12:35 UT) we began observing the field of GRB 080805
(Pagani et al. GCN Circ. 8059
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 ) with the Faulkes Telescope South using
the R and i' filters.

In our coadded i' frame we marginally detect the optical afterglow
(Kruehler et al. GCN Circ. 8060
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 ; de Ugarte Postigo et al. GCN Circ. 8061
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 ):

Telescope   Filter  T_mid[hr]   Exposure[s]      Mag
-----------------------------------------------------------
FTS         i'         5.410       6x300       22.5 +/- 0.4
-----------------------------------------------------------

The calibration was performed using the I magnitudes of several
USNOB-1 catalogue field objects.
From the comparison with the value reported by GROND
(Kruehler et al. GCN Circ. 8060
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 ) we estimate an average
power-law decay index of ~0.7, consistent with that inferred
in the R band (de Ugarte Postigo et al. GCN Circ. 8061
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 ).

C. Pagani, J. Racusin and D. Grupe (PSU) report on behalf of the Swift-XRT team:

We have analysed the first four orbits of Swift-XRT data for GRB 080805 (Pagani
et al., GCN 8059
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 ) beginning at T0+71 s until T0+23.1 ks.

Using 728 s of XRT Photon Counting mode data and 2 UVOT images, 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 = 314.22312,
-62.44479 which is equivalent to:

RA (J2000):  20 56 53.55
Dec (J2000): -62 26 41.2

with an uncertainty of 2.6 arcsec (radius, 90% confidence).

The XRT lightcurve shows an initial flare and a steep decay followed by a
shallower phase. The decaying lightcurve can be fit with a broken power-law
with an initial decay index of 3.5+/-0.3, a break at approximately T0+500
seconds and a later decay index of 0.9+/-0.3.

The spectrum of the Windowed Timing mode data from the first orbit can be fit by
an absorbed power-law with a photon index of 1.3+/-0.1 and absorption consistent
with the Galactic value along the line of sight (3.5e20 cm^-2, Kalberla et al.
2005).  The average observed (unabsorbed) flux of the WT mode data is 1.7e-9
(1.9e-10) ergs 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 0.003 counts/s at T+24hr, which corresponds to
an observed flux of 1.5e-13 ergs cm^-2 s^-1.

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

P. Ward (MSSL-UCL) and C. Pagani (PSU) report on behalf of
the Swift/UVOT team:

The Swift/UVOT observed the field of GRB 080805 starting 74 s after the 
BAT trigger (Pagani et al., GCN Circ. 8059
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 ).  We do not find a source in 
any of the UVOT observations inside the XRT error circle or at the 
position reported by Kruehler et al. (GCN 8060
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 ). The 3-sigma upper limits 
for detecting a source are:

Filter   T_start T_stop   Exp   Mag

White    76      175      98.5  > 20.70
White    5473    18024    1082  > 22.12
v        182     6083     332   > 19.81
b        5268    17112    1082  > 21.43
u        5061    5260     197   > 20.11
uvw1     4856    5055     197   > 19.99
uvm2     4650    6267     371   > 20.09
uvw2     5680    18053    219   > 20.12

The values quoted above are in the UVOT photometric system
(Poole et al. 2008, MNRAS, 383, 627). No correction has been
made for the expected extinction corresponding to E(B-V) of
0.043 mag in the direction of the burst (Schlegel et al. 1998).

D. Palmer (LANL), 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), K. McLean (GSFC/UMD),
C. Pagani (PSU), T. Sakamoto (GSFC/UMBC), G. Sato (ISAS),
M. Stamatikos (GSFC/ORAU), J. Tueller (GSFC), T. Ukwatta (GWU)
(i.e. 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 080805 (trigger #319036)
(Pagani, et al., GCN Circ. 8059
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 ).  The BAT ground-calculated position is
RA, Dec = 314.231, -62.433 deg, which is 
   RA(J2000)  =  20h 56m 55.3s 
   Dec(J2000) = -62d 25' 58.8" 
with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 100%.
 
The mask-weighted light curve shows a FRED-like peak starting at ~T-5sec,
peaking at ~T+3 sec, and ending at ~T+140 sec.  T90 (15-350 keV) is
78 +- 7 sec (estimated error including systematics).
 
The time-averaged spectrum from T-6.1 to T+93.3 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.53 +- 0.07.  The fluence in the 15-150 keV band is 2.5 +- 0.1 x 10^-6 erg/cm2.
The 1-sec peak photon flux measured from T+2.62 sec in the 15-150 keV band
is 1.1 +- 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/319036/BA/

M.Cwiok, W.Dominik, G.Kasprowicz, A.Majcher, A.Majczyna,
K.Malek, L.Mankiewicz, M.Molak, K.Nawrocki, L.W.Piotrowski,
D.Rybka, M.Sokolowski, J.Uzycki, G.Wrochna, A.F.Zarnecki
on behalf of "Pi of the Sky" collaboration http://grb.fuw.edu.pl

"Pi of the Sky" apparatus observed the position of GRB080805. Three 10s
images of this field have been taken at 07:37:45 - 07:38:42 ( 3 minutes
before the GRB). No new object is seen within the error box and the limit
is 12.5 magnitudo.

After receiving the trigger from SWIFT, the field was observed 84s after
the GRB. No new object has been found within the Swift-BAT error box.
The limiting magnitude (unfiltered) on single 10s exposures is :

         07:42:58 - 07:43:08 - 12.2 mag
         07:43:11 - 07:43:21 - 12.2 mag
         07:43:24 - 07:43:34 - 12.2 mag

  and 12.5 mag on 20 coadded images.

P. Jakobsson (U. Hertfordshire), P. M. Vreeswijk (DARK/NBI),
A. de Ugarte Postigo (ESO), D. Malesani, J. P. U. Fynbo (DARK/NBI)
and J. Sollerman (DARK/NBI; U. Stockholm) report on behalf of a
larger collaboration:

Using FORS2 on the ESO Very Large Telescope, we have obtained
low and medium resolution spectra (grisms 300V, 1200R and 1400V)
of the optical afterglow of GRB 080805 (Kruehler et al., GCN 8060
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 ;
de Ugarte Postigo et al., GCN 8061
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 ). The first spectrum was
taken approximately an hour after the burst.

A firm upper limit of z < 2.2 can be placed on the redshift
of GRB 080805 from the lack of Ly-alpha forest lines in the
spectrum of the afterglow. This is consistent with the GROND
observations (Kruehler et al).

The spectra display Fe II (2344,2382,2600) and the Mg II
doublet (2796,2803) in absorption corresponding to a redshift
of z = 1.20. There is also tentative evidence of an absorption
system at z = 1.50 (Fe II and Mg II), but further analysis is
required to confirm the reality of this system.

We thank the Paranal staff for excellent support, especially
Patricia Guajardo and Yuri Belestky.

A. de Ugarte Postigo (ESO), D. Malesani, P. M. Vreeswijk,
J.P.U. Fynbo (DARK/NBI), P. Jakobsson (Univ. Hertfordshire),
A.O. Jaunsen (Univ. Oslo), J. Sollerman (DARK/NBI and Univ.
Stockholm), report:

We observed the field of GRB 080805 (Pagani et al., GCN 8059
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 )
with the ESO VLT equipped with FORS2. We clearly identify the
candidate afterglow first reported by Kruehler et al. (GCN 8060
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 ).
From the first image, obtained at 07:50:49 UT (9.3 min after the
burst) to 10:00:05 UT (138.5 min after the burst) the object faded
by 2.0 magnitudes in the R band. We thus consider the afterglow
candidate to be secure.

We acknowledge excellent support from the observing staff at
Paranal, especially to Patricia Guajardo and Yuri Belestky.

T. Kruehler, F. Schrey, J. Greiner, C. Clemens, A. Yoldas (all MPE 
Garching),
A. Kupcu Yoldas (ESO) and G. Szokoly (Eoetvoes Univ., Budapest) report on
behalf of the GROND team:

GROND (Greiner et al. 2008, PASP 120, 405), mounted at the 2.2m ESO/MPI 
telescope at La Silla Observatory (Chile), started observations of the 
field of GRB 080805 (Pagani et al. 2008, GCN #8059
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 ) in g'r'i'z'JHK at 
07:45 UT, 3.5 min after the burst. We detect an optical/NIR afterglow 
candidate in the XRT error circle (Pagani et al. 2008, GCN #8059
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 ) in 
griz at:

RA(J2000): 20h 56m 53.44
DEC(J2000): -62d 26' 39.8"

with uncertainties of 0.5".

We measure the following magnitudes, calibrated against USNO-B1 field stars:

g = 21.7 +- 0.2
r = 20.5 +- 0.1
i = 20.1 +- 0.1
z = 19.9 +- 0.1

No statement about fading can be made at the moment. If this is the
afterglow of GRB 080805, then its redshift is smaller than 3.5.

C. Pagani (PSU), W. H. Baumgartner (GSFC/UMBC), M. M. Chester (PSU),
J. R. Cummings (NASA/UMBC), P. A. Evans (U Leicester),
D. Grupe (PSU), C. Guidorzi (INAF-OAB),
S. T. Holland (CRESST/USRA/GSFC), J. Mao (INAF-OAB),
A. Moretti (INAF-OAB), K. L. Page (U Leicester), D. M. Palmer (LANL),
J. L. Racusin (PSU), P. Romano (INAF-IASFPA) and
H. Ziaeepour (UCL-MSSL) report on behalf of the Swift Team:

At 07:41:34 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 080805 (trigger=319036).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 314.231, -62.438 which is 
   RA(J2000) = 20h 56m 55s
   Dec(J2000) = -62d 26' 16"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a FRED-like
structure with a duration of about 50 sec.  The peak count rate
was ~2600 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 07:42:38.3 UT, 63.5 seconds after
the BAT trigger. Using promptly downlinked data we find a bright, fading
uncatalogued X-ray source located at RA, Dec 314.22273, -62.44433 which
is equivalent to:
   RA(J2000)  = 20h 56m 53.46s
   Dec(J2000) = -62d 26' 39.6"
with an uncertainty of 3.6 arcseconds (radius, 90% containment).  This position
is 26.6 arcseconds from the BAT position, within the BAT 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
3.45e+20 cm^-2 (Kalberla et al. 2005). 

UVOT took a finding chart exposure of 100 seconds with the White 
(160-650 nm)  filter starting 74 seconds after the BAT trigger. No 
afterglow candidate is apparent in the initial data products. The 
2.7'x2.7' sub-image covers 25% of  the BAT error circle and 100% of 
the XRT error circle. The typical 3-sigma upper limit has been about 
 18.5 mag. The list of sources is typically complete to about 18 mag. 
 No  correction has been made for the expected extinction
corresponding   to E(B-V) of 0.043. 

Burst Advocate for this burst is C. Pagani (pagani 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.)