GCN Circular 8760

GRB 081230: GROND confirmation of the afterglow 


P. Afonso (MPE), C. Clemens (MPE), S. Klose (Tautenburg), J. Greiner
(MPE) and G. Szokoly (Eotvos 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 081230
(Swift trigger 338633, La Parola et al., GCN #8753) simultaneously in
g'r'i'z'JHK bands 

We detected and confirm the afterglow candidate in all bands, at the 
position: 

RA(J2000): 02:29:19.51
DEC(J2000): -25:08:49.95 

with uncertainties of 0.5" in each coordinate. 

This position is compatible with UVOT (Oates et alia, GCN #8757) and
ROTSE-III (Yuan et alia, GCN #8754) reported positions. 

Observations started on Dec. 31 at 00h48 UT, 15123 seconds, or ~ 4.2 hours
after the burst. 

We used 4 images of 375 s exposure in g'r'i'z' and 120 images of 10s
exposure in JHK, for each epoch. 

The images mid time for the 1st epoch is 5.99h (or 21564 seconds) after the 
burst. Observations were done at an average air mass = 1.128 and seeing ~ 
1.5''. 

We took a 2nd epoch on January 1st, 2009, starting at 03h21 UT. 

The images mid time for the 2nd epoch is 30.99h (or 111564 seconds) after 
the burst. Observations were done at an average air mass = 1.334 and seeing 
~ 1.3''. 

The preliminary data for the 2nd epoch clearly confirms the fading of the 
afterglow: 

We present preliminary photometric results for g'r'i'z'. The AB magnitudes, 
using GROND zero points are as follows: 

     1st epoch       2nd epoch 

g' = 21.32 +- 0.02   23.67 +- 0.08
r' = 20.96 +- 0.01   23.07 +- 0.06
i' = 20.74 +- 0.02   22.79 +- 0.09
z' = 20.66 +- 0.03   22.84 +- 0.17 

No correction for galactic extinction was done. 

The quoted error is statistical only. There is an additional
systematic error in the absolute calibration using the GROND zeropoints
which is expected to be in the 0.2 mag range. 

Further data analysis is ongoing for JHK - the aftegrlow being clearly 
visible on the first epoch, but not on the 2nd.