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.