GCN Circular 8752

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).

The optical/NIR source inside the XRT errorcircle, previously reported 
in GCN #8745 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.