TITLE: GCN CIRCULAR NUMBER: 10299 SUBJECT: GRB 091230: GROND optical/NIR afterglow candidate DATE: 09/12/30 13:00:36 GMT FROM: Thomas Kruehler at MPE/MPI R. Filgas, T. Kruehler and J. Greiner (all MPE Garching) report on behalf of the GROND team: We observed the field of GRB 091230 (INTEGRAL trigger #5967, D. Gotz et al., GCN #10298) simultaneously in g'r'i'z'JHK with GROND mounted at the 2.2m ESO/MPI telescope at La Silla Observatory (Chile). Observations started on 30 Dec. 2009 at 06:30:27 UT, 2.9 min after the burst. Image subtraction reveals a variable point source inside the INTEGRAL error circle (D. Gotz et al., GCN #10298) at RA (J2000.0) = 08:51:39.18 DEC (J2000.0) = -53:53:52.7 with an uncertainty of 0.5" in each coordinate. This is 56" from the refined INTEGRAL position. Preliminary photometry yields the following magnitudes in stacked images: Filter T_mid[s] Exp[s] AB Mag MagErr --------------------------------------- g' 390 4x 66 21.63 0.07 r' 390 4x 66 20.76 0.04 i' 390 4x 66 20.45 0.05 z' 390 4x 66 20.22 0.05 J 403 24x 10 19.75 0.12 H 403 24x 10 19.58 0.14 K 403 24x 10 19.26 0.15 The quoted error is statistical only. There is an additional systematic error in the absolute calibration using the GROND zeropoints and 2MASS field stars which is expected to be in the 0.1 to 0.2 mag range. The object fades by two magnitudes to r' = 22.8 ± 0.1 until the end of our optical observations two hours after the trigger - thus we propose this object as the optical/NIR afterglow of GRB 091230.