TITLE: GCN CIRCULAR NUMBER: 13926 SUBJECT: GRB 121027A: GROND Confirmation of Rebrightening DATE: 12/10/29 19:48:09 GMT FROM: Alexander Kann at TLS Tautenburg V. Sudilovsky (MPE Garching), D. A. Kann (MPE Garching/Universe Cluster), T. Krühler (DARK, Copenhagen), and J. Greiner (MPE Garching) report on behalf of the GROND team: Following two days of bad weather delay, we observed the field of GRB 121027A (Evans et al., GCN 13906) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP, 120, 405) mounted on the 2.2 m MPG/ESO telescope at La Silla Observatory (Chile). Observations began at 08:10 on October 29th (mid-time ~2.05 days post-trigger). They were performed at an average seeing of 1.6" (r'-band) and at an average airmass of 1.2. We detect the NIR afterglow reported by Starling et al. (GCN 13911) and Levan et al. (GCN 13920) in all seven bands. Based on a 460 sec integration in g'r'i'z' and 1200 sec integration in JHK, we measure preliminary magnitudes (all in AB system) of g' = 22.1 +/- 0.2 mag r' = 21.6 +/- 0.1 mag i' = 21.3 +/- 0.1 mag z' = 20.9 +/- 0.1 mag J = 20.4 +/- 0.1 mag H = 19.7 +/- 0.1 mag K = 19.0 +/- 0.1 mag We note an increase of brightness by ~ 1 magnitude relative to the observations reported by Levan et al. (GCN 13920) at 32 hours after the GRB. Our detection in all bands constrains the redshift of this burst to z < 3.5. Given magnitudes are calibrated against GROND zeropoints in the optical, and 2MASS field stars in the NIR, and are not corrected for the expected Galactic foreground extinction corresponding to a reddening of E(B-V) = 0.02 mag in the direction of the burst (Schlegel et al. 1998).