{
  "bibcode": "2008GCN..8752....1A",
  "body": "P. Afonso, T. Kruehler, J. Greiner (all MPE), and S. Klose \n(Tautenburg) report on behalf of the GROND team:\n\nWe report on further analysis of the GROND data of the optical \nafterglow candidate of GRB 081228 (Swift trigger 338338, Page et al., \nGCN #8742).\n\nThe optical/NIR source inside the XRT errorcircle, previously reported \nin GCN #8745 by P. Afonso et al., is found to be clearly variable. The \nposition of the afterglow is:\n\nRA(J2000): 02:37:50.94\nDEC(J2000): +30:51:10.5\n\nwith uncertainties of 0.5\" in each coordinate.\n\nPreliminary photometry yields the following r' band magnitudes, \nuncorrected for Galactic foreground reddening:\n\nT_mid[s]  Exp[s] Mag     MagErr\n-------------------------------\n449       35     19.81   0.04\n667       35     20.25   0.04\n872       35     20.59   0.06\n1380      115    21.21   0.05\n1581      115    21.34   0.05\n3308      375    22.09   0.08\n\nThe quoted error is statistical only. There is an additional \nsystematic error in the absolute calibration using the GROND zeropoint \nwhich is expected to be in the 0.2 mag range. As the GROND photometric \nsystem is based on the SDSS in g'r'i'z' these values are AB magnitudes.\n\nThe complete light curve obtained during the first night post burst \ncan be reasonably well described with a power law of index 1.1 +- 0.2.\n\nA broad band SED from g' to K was constructed using data obtained \nsimultaneously at a midtime of 1500 s after the trigger. After \ncorrecting for a Galactic foreground extinction corresponding to a \nreddening of E_(B-V)= 0.16 (Schlegel et al. 1998), the r' to K band \nSED is well described with a power law of spectral index beta = 0.85 \n+- 0.15.\n\nWe tentatively associate the large, foreground extinction corrected \ng'-r' color of around 1.4 mag with Ly-alpha absorption in the host at \na redshift of 3.8 +- 0.4. This photometric redshift estimate has been \nobtained using hyperZ (Bolzonella et al. 2000). We caution, however, \nthat this might be subject to changes due to a future improved \ncalibration.",
  "circularId": 8752,
  "createdOn": 1230575373000,
  "email": "kruehler@mpe.mpg.de",
  "subject": "GRB 081228: GROND confirmation of the optical/NIR afterglow",
  "submitter": "Thomas Kruehler at MPE/MPI  <kruehler@mpe.mpg.de>",
  "eventId": "GRB 081228"
}