GRB 081204

A. Updike (Clemson University), C. Clemens, and J. Greiner (both MPE
Garching) report on behalf of the GROND team:

We observed the field of GRB 081204 (INTEGRAL trigger #5385; D. Gotz et
al., GCN #8614
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 ) for two epochs simultaneously in g'r'i'z'JHK with GROND
(Greiner et al. 2008, PASP 120, 405) mounted at the 2.2 m ESO/MPI
telescope at La Silla Observatory (Chile).

Observations began at 00:30 UT on Dec 5, 2008, 7.75 hr after the GRB
trigger, and consisted of 2.4 hours of exposures.  The second epoch began
at 00:31 UT on Dec 6, 31.75 hr after the trigger, and consisted of 1.3
hours of exposures. Observations were performed under favorable sky
conditions.

We detect two sources within the refined 5.3'' Swift-XRT error circle
reported by V. Mangano et al. (GCN #8620
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 ), at the astrometrically
corrected positions:

RA (J2000.0) 23h 19m 09.10s
Dec (J2000.0) -60d 13' 29.7"

and

RA (J2000.0) 23h 19m 09.37s
Dec (J2000.0) -60d 13' 31.2"

The second candidate agrees with the position reported by Berger & Rest
(GCN #8624
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 ).

We detect no evidence of fading in either source between the two epochs.
Therefore, we suggest that neither of the sources are the afterglow of
GRB 081204.

Our upper limits from the first epoch in g'r'i'z'JHK are as follows:

g' > 24.2
r' > 24.3
i' > 23.6
z' > 22.9
J  > 20.7
H  > 19.4
K  > 18.3

The given magnitudes are calibrated against GROND zeropoints as well as
2MASS field stars.

E. Berger and A. Rest (Harvard) report:

"We observed the position of GRB 081204 (GCN 8614
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 ) with the LDSS3
instrument on the Magellan/Clay telescope.  A total of 30 sec were
obtained in r- and i-band on 2008 Dec 5.067 UT (8.86 hour after the
burst).  Within the XRT error circle (GCN 8620
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 ) we detect a single faint
source with r=23.5+/-0.3 mag and i=23.0+/-0.2 mag located at:
	RA  = 23:19:09.41
	DEC = -60:13:31.4  (J2000)
with an uncertainty of about 0.3" in each coordinate (astrometry relative
to USNO-B).  At the moment we have no information on whether the source is
variable."

P.A. Curran (UCL-MSSL) and W. Baumgartner  (GSFC)
report on behalf of the Swift/UVOT team:


The Swift Ultra-Violet/Optical Telescope (UVOT) began settled ToO 
observations of the  INTEGRAL-IBAS localised  GRB 081204 (Gotz et al., 
GCN 8614
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 ) ~ 3 hours after the event. The field was observed with the 
white filter only. No afterglow is detected within the 7" XRT position 
(Mangano et al., GCN 8616
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 ) in the co-added image, down to the following 
3-sigma limiting magnitude:

Filter      T_start(s)    Exp(s)     Mag UL (3sig)
-----------------------------------
white      1.01e4        3.4e3      >  22.04
-----------------------------------

There is a DSS detected object approximately 11" south-west of the 
center of the XRT position with an observed white magnitide of ~20.5.

The values quoted above are in the UVOT photometric system (Poole et al. 
2008, MNRAS, 383, 627); they are not corrected for the expected Galactic 
extinction along the line of sight which corresponds to a reddening of 
E(B-V)=0.027 mag (Schlegel et al. 1998).

[GCN OPS NOTE(05dec08): Per author's request, the typo for T_start
in the table was changed from 1.01e5 to 1.01e4.]

V. Mangano, B. Sbarufatti, V. La Parola (INAF IASF PA),
and W. H. Baumgartner (GSFC/UMBC)
report on behalf the Swift XRT team:

The Swift ToO observation of GRB 081204 started on 2008 Dec 4th
at 19:32 UT consists of 3.6 ks XRT exposure in PC mode.
We confirm the detection of the previously reported afterglow
candidate (Mangano et al., GRC Circ. #8616) at the refined position
RA, Dec = 349.7880, -60.2255 which is equivalent to:

RA (J2000):   23 19 09.13 
Dec (J2000): -60 13 31.7

with an uncertainty of 5.3 arcsec (radius, 90% confidence).
This is 30 arcsec from the INTEGRAL position (Gotz, GCN Circ. #8614
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 ).
The source is fading with a power law decay index of about -2,
and will reach a level of about 5.0e-4 counts/s in 24 hours
from the trigger.

The average spectrum accumulated in the time interval from
10 ks to 37 ks after the event was fitted, using Cash
statistics, with an absorbed power law. The best fit photon
index is 2.7 � 1.5. The spectrum shows evidence of absorption
at the level of (4.4 � 4.0)e21 cm-2, in excess of the
estimated Galactic value (1.89e+20 cm-2, Kalberla et al. 2005). 
The average observed[unabsorbed] 0.3-10 keV average flux is
about  2.7[8.5]e-13 ergs cm-2 s-1.
The count-rate-to-flux conversion factor is 1.5e-10.

All quoted errors are at 90% confidence level.

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

V. Mangano, B. Sbarufatti, V. La Parola (INAF-IASF PA)
report on behalf the Swift XRT team:

We have analyzed the first orbit data (632 s exposure) of the
Swift ToO observation of the INTEGRAL detected burst GRB 081204
(Gotz, GCN Circ. #8614
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 ).
The Swift observation started on 2008 Dec 04 at 19:29 UT,
about 2h 45m after the INTEGRAL trigger.
We detected a single X-ray source within the INTEGRAL error box at
RA, Dec =349.7873, -60.2253 which is equivalent to:

RA (J2000):  23 19 08.96
Dec (J2000): -60 13 31.3

with an uncertainty of 7 arcsec (radius, 90% confidence).
The source lies 29 arcsec from the INTEGRAL position and has
a count rate of ~2.0E-02 counts/s with a 2.6 sigma significance.
At the moment it is not possible to determine whether it is fading,
but the Swift observation is still on going and we are waiting for
more data.

This is an official product of the Swift XRT team.

D. Gotz (CEA-Saclay), S.Mereghetti, A.Paizis (IASF-Milano), R. Galis,
V.Beckmann, M. Beck (ISDC, Versoix), and J. Borkowski (CAMK, Torun) on
behalf of the IBAS Localization Team report:

A GRB lasting about 20 s has been detected by IBAS in IBIS/ISGRI data at
16:44:55 UT on December 4th. The refined coordinates (J2000)  are:

RA = 349.7735 [degrees]
DEC= -60.2213 [degrees]

with an uncertainty of 2.3 arcmin (90% c.l.).

A preliminary analysis gives a peak flux in the 20-200 keV range of  
about
0.7 ph/cmq/s (1-s integration time) and a fluence over the same energy
range of about 7x10e-7 erg/cmq.

A plot of the light curve will be posted at

http://ibas.iasf-milano.inaf.it/IBAS_Results.html

This message can be cited.

[GCN OPS NOTE(04dec08): Per author's request, the second "GRB" was added
to the Subject line.]