TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3919 SUBJECT: GRB 050904: SOAR YJ and PROMPT Ic Observations DATE: 05/09/05 07:04:32 GMT FROM: Daniel E. Reichart at U.North Carolina J. Haislip, M. Nysewander, D. Reichart, E. Cypriano, D. Maturana, S. Pizarro, C. MacLeod, J. Kirschbrown, E. Figueredo report on behalf of the UNC team of the FUN GRB Collaboration. We continued observations of the afterglow (Haislip et al., GCN 3913) of GRB 050904 (Cummings et al., GCN 3910) with SOAR and PROMPT at CTIO in YJ and Ic, respectively, beginning 26.4 hours after the burst. We detect the afterglow in J and refine the fading rate to be -1.00 +/- 0.12. We also detect the afterglow in Y, which suggests that the redshift is at the lower end of the redshift ranges of Haislip et al. (GCN 3914) and Reichart (GCN 3915). Furthermore, a redshift of 6 (+/- 1) appears to be consistent with the relatively faint, unfiltered optical detections of Klotz et al. (GCN 3917) from only minutes after the burst: Extrapolation of our J-band light curve back to this time suggests that the afterglow was probably relatively bright (J ~ 13 mag) redward of the Ly-alpha forest. Integration of the light across a broad optical spectral response curve would then result in significantly fainter magnitudes for such redshifts.