GRB 081228

K. L. Page (U Leicester), W. H. Baumgartner (GSFC/UMBC),
P. J. Brown (PSU), D. N. Burrows (PSU),
S. T. Holland (CRESST/USRA/GSFC), J. A. Kennea (PSU),
H. A. Krimm (CRESST/GSFC/USRA), C. B. Markwardt (CRESST/GSFC/UMD) and
T. N. Ukwatta (GSFC/GWU) report on behalf of the Swift Team:

At 01:17:40 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 081228 (trigger=338338).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 39.467, +30.843 which is 
   RA(J2000) = 02h 37m 52s
   Dec(J2000) = +30d 50' 33"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single peak
with a duration of about 2.5 sec.  The peak count rate
was ~1500 counts/sec (15-350 keV), at ~0.5 sec after the trigger. 

The XRT began observing the field at 01:18:58.7 UT, 78.0 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 39.46043, 30.85393 which is equivalent
to:
   RA(J2000)  = 02h 37m 50.50s
   Dec(J2000) = +30d 51' 14.1"
with an uncertainty of 4.8 arcseconds (radius, 90% containment). This
location is 44 arcseconds from the BAT onboard position, within the BAT
error circle. This position may be improved as more data are received;
the latest position is available at http://www.swift.ac.uk/sper. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of
8.8e+20 cm^-2 (Kalberla et al. 2005). 

UVOT took a finding chart exposure of  37 seconds with the White filter
starting 82 seconds after the BAT trigger. No credible afterglow candidate has
been found in the initial data products. The 2.7'x2.7' sub-image covers 25% of
the BAT error circle. The typical 3-sigma upper limit has been about 19.6 mag. 
The 8'x8' region for the list of sources generated on-board covers 100% of the
BAT error circle. 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
0.16. 

Burst Advocate for this burst is K. L. Page (kpa AT star.le.ac.uk). 
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.)

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

We imaged the field of GRB 081228 detected by SWIFT
(trigger 338338) with the TAROT robotic telescope (D=25cm)
located at the Calern observatory, France.

The observations started 24.7s after the GRB trigger
(6.1s after the notice). The elevation of the field decreased
from 24 degrees above horizon and weather conditions
were good.

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 in the XRT error box (Page et al.
GCNC 8742) with a limiting magnitude of:
t0+24.7s to t0+84.7s : R > 16.9

The second image is 30.0s exposure in tracking mode:
t0+91.8s to t0+121.8s : R > 17.4

We co-added a series of exposures:
t0+91.8s to t0+384.6s : R > 18.4

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

N.B. Galactic coordinates are lon=148.4290 lat=-26.7069
and the galactic extinction in R band is 0.4 magnitude
estimated from D. Schlegel et al. 1998ApJ...500..525S.

This message may be cited.

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

Using 27 s of XRT Photon Counting mode data and 1 UVOT
images for GRB 081228, 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 = 39.4616, +30.8526 which is equivalent
to:

RA (J2000): 02h 37m 50.77s
Dec (J2000): +30d 51' 09.5"

with an uncertainty of 2.9 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.

GRB 081228: detection by GROND 

P. Afonso (MPE), S. Klose (Tautenburg), T. Kruehler, (MPE), J. Greiner
(MPE), A. Updike (Clemson University), A. Kupcu-Yoldas(ESO), A. Yoldas 
(MPE), and G. Szokoly (E��tv��s 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) imaged the field of GRB 081228
(Swift trigger 338338, Page et al., GCN #8742
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 ) simultaneously in
g'r'i'z'JHK. 

Inside the enhanced XRT error circle (position, RA (J2000): 02h 37m 50.77s
Dec (J2000): +30d 51' 09.5"), reported by Goad et alia, GCN #8744
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 , we
detect an object in the g'r'i'z' bands, but not in JHK. 

Observations started at 01h26 UT, 9 min after the burst. 

We used 4 images of 375 s exposure in g'r'i'z' and 120 images of 10s
exposure in JHK. The images midtimes for g'r'i'z' and JHK are respectively
02h23m59s and 02h24m20s UT, or 1h6m19s and 1h6m40s after the burst. 

Observations were done at high air mass and seeing bigger than 1''. 

The object is detected in the g'r'i'z' bands, implying a redshift smaller
than 3.5. The detection is fainter in the g' band. 

Preliminary VEGA magnitudes calibrated against USNO-B field stars and
upper limits are as follows: 

g' = 23.73 +/- 0.11
r' = 21.98 +/- 0.04
i' = 21.42 +/- 0.06
z' = 21.80 +/- 0.08
J > 19.6
H > 18.4
K > 17.6 

No correction for galactic extinction was done.

K.L. Page & P.A. Evans (U. Leicester) report on behalf of the Swift-XRT 
team:

We have analysed the first 4.6 ks of XRT data obtained for GRB 081228 
(Page et al., GCN Circ. 8742
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 ), all of which was collected in Photon 
Counting mode. We have further refined the position given by Goad et al. 
in GCN Circ. 8744
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 , and now find an astrometrically corrected X-ray 
position (using the XRT-UVOT alignment and matching UVOT field sources to 
the USNO-B1 catalogue) of

RA (J2000):   02 37 50.89
Dec (J2000): +30 51 09.1

with an uncertainty of 2.2 arcsec (radius, 90% confidence).

The data can be fitted with a single power-law with a decay of alpha = 1.4 
+/- 0.1, although there is some evidence for a flattening after about 8 
ks.

The spectrum of the first orbit of data can be modelled with a power-law 
of photon index Gamma = 1.5 +/- 0.4, absorbed by the Galactic column in 
this direction of 8.8x10^20 cm^-2. The upper limit on the total absorbing 
column is 4.7x10^21 cm^-2. The observed (unabsorbed) flux over this time 
(86-117 s after the trigger) is 1.23x10^-10 (1.38x10^-10) erg cm^-2 s^-1, 
corresponding to a counts to observed flux conversion of 5.1x10^-11 erg 
cm^-2 count^-1.

Taking into account the possible flattening of the decay at later times, 
the count rate at 24 hours is predicted to be between 4.5x10^-3 and 
2.7x10^-4 count s^-1 (observed flux between 2.3x10^-13 and 1.4x10^-14 erg 
cm^-2 s^-1).

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

W. Landsman (GSFC) and K. L. Page (U. Leicester) report on behalf of the 
Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 081228 82s 
after the BAT trigger (Page et al., GCN 8742
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 ).
The exposure time was limited by observing constraints until about 1 
hour after the trigger.   We do not detect any source at the refined 
Swift XRT position (Page & Evans GCN 8746
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 ).   UVOT magnitude 3-sigma 
upper limits in a 5" aperture are reported in the following table:

Filter   T_start     Exp(s)  Mag (3-sigma upper limit)
------------------------------------------------------
white       82         37      > 20.0
white     4679        137      > 20.7
v         3653        197      > 19.3
b         4473        197      > 20.3
u         4268        197      > 20.0
uvw1      4063        197      > 20.0
uvm2      3858        197      > 20.0
uvw2      9102        886      > 20.9

The quoted upper limits have not been corrected for the expected 
Galactic extinction along the line of sight of E_(B-V) = 0.16 mag.   All 
photometry is on the UVOT photometric system described in Poole et al. 
(2008, MNRAS, 383, 627).

C. B. Markwardt (GSFC/UMD), S. D. Barthelmy (GSFC), W. H. Baumgartner 
(GSFC/UMBC),
J. R. Cummings (GSFC/UMBC), E. E. Fenimore (LANL), N. Gehrels (GSFC),
H. A. Krimm (GSFC/USRA), K. McLean (GSFC/UMD), K. L. Page (U Leicester),
D. M. Palmer (LANL), A. M. Parsons (GSFC), T. Sakamoto (GSFC/UMBC),
G. Sato (ISAS), M. Stamatikos (GSFC/ORAU), J. Tueller (GSFC),
T. N. Ukwatta (GWU) (i.e. the Swift-BAT team):

Using the data set from T-239 to T+963 sec from the recent telemetry 
downlink,
we report further analysis of Swift GRB 081228 (trigger #338338)
(Page, et al., GCN Circ. 8742
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 ).  The BAT ground-calculated position is
RA, Dec = 39.477, 30.833 deg which is
  RA(J2000)  =  02h 37m 54.4s
  Dec(J2000) = +30d 49' 57.3"
with an uncertainty of 2.3 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 85%.

The mask-weighted light curve begins with a possible faint soft 20-second
precursor beginning at T-180 sec.  The main emission consists of a single
peak beginning at T-0.5 sec and lasting ~2.5 seconds. T90 (15-350 keV) is
3.0 +- 1.4 sec (estimated error including systematics). An observing 
constraint
caused the Swift spacecraft to slew away from the burst location at
~T+250 seconds, so no information about the burst is available after
~T+300 sec, when the burst was no longer in the field of view.

The time-averaged spectrum from T-0.1 to T+2.9 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
2.10 +- 0.31.  The fluence in the 15-150 keV band is 8.9 +- 1.8 x 10-8 
erg/cm2.
The 1-sec peak photon flux measured from T+0.87 sec in the 15-150 keV band
is 0.6 +- 0.1 ph/cm2/sec.  All the quoted errors are at the 90% confidence
level.

With a duration longer than 2 s and a soft spectrum, this burst is 
likely to
be in the "long" category.

The results of the batgrbproduct analysis are available at
http://gcn.gsfc.nasa.gov/notices_s/338338/BA/

P. Afonso, T. Kruehler, J. Greiner (all MPE), and S. Klose 
(Tautenburg) report on behalf of the GROND team:

We report on further analysis of the GROND data of the optical 
afterglow candidate of GRB 081228 (Swift trigger 338338, Page et al., 
GCN #8742
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 ).

The optical/NIR source inside the XRT errorcircle, previously reported 
in GCN #8745
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  by P. Afonso et al., is found to be clearly variable. The 
position of the afterglow is:

RA(J2000): 02:37:50.94
DEC(J2000): +30:51:10.5

with uncertainties of 0.5" in each coordinate.

Preliminary photometry yields the following r' band magnitudes, 
uncorrected for Galactic foreground reddening:

T_mid[s]  Exp[s] Mag     MagErr
-------------------------------
449       35     19.81   0.04
667       35     20.25   0.04
872       35     20.59   0.06
1380      115    21.21   0.05
1581      115    21.34   0.05
3308      375    22.09   0.08

The quoted error is statistical only. There is an additional 
systematic error in the absolute calibration using the GROND zeropoint 
which is expected to be in the 0.2 mag range. As the GROND photometric 
system is based on the SDSS in g'r'i'z' these values are AB magnitudes.

The complete light curve obtained during the first night post burst 
can be reasonably well described with a power law of index 1.1 +- 0.2.

A broad band SED from g' to K was constructed using data obtained 
simultaneously at a midtime of 1500 s after the trigger. After 
correcting for a Galactic foreground extinction corresponding to a 
reddening of E_(B-V)= 0.16 (Schlegel et al. 1998), the r' to K band 
SED is well described with a power law of spectral index beta = 0.85 
+- 0.15.

We tentatively associate the large, foreground extinction corrected 
g'-r' color of around 1.4 mag with Ly-alpha absorption in the host at 
a redshift of 3.8 +- 0.4. This photometric redshift estimate has been 
obtained using hyperZ (Bolzonella et al. 2000). We caution, however, 
that this might be subject to changes due to a future improved 
calibration.