GCN Circular 12656
D. A. Kann (TLS Tautenburg), and J. Greiner, (MPE Garching) report on behalf of the GROND team: We have analyzed the J-band observations (Kann et al., GCN #12647) obtained with GROND over a total of five hours (0.74 to 0.93 days after the trigger) of GRB 111209A (Hoversten et al., GCN #12632). Astrometry and calibration were obtained against the 2MASS catalog. We find the afterglow experiences a strong rise during our observations, rising from J ~ 19.3 to J ~ 18.8 (AB Magnitudes). The slope of the rise is alpha < -2 (assuming F(t) propto t^-alpha). This behavior may be due to a strong energy injection. Highly variable behavior in the first hours has already been reported by Klotz et al. (GCN #12637) and Hoversten & Siegel (GCN #12642). Dense photometric follow-up is strongly encouraged.