TITLE: GCN CIRCULAR NUMBER: 9217 SUBJECT: GRB 090423: Deep z-band upper limit and confirmation of photo-z DATE: 09/04/24 03:51:52 GMT FROM: Daniel Perley at U.C. Berkeley D. A. Perley, S. B. Cenko, and J. S. Bloom (UC Berkeley) report on behalf of GRAASP: We initiated a series of 15 x 200 second exposures of the field of GRB 090423 (Krimm et al, GCN 9198) in z-band using GMOS on Gemini-South starting at 23:38 UT on 2009-04-23, 15.71 hours after the trigger. No source is visible in the XRT error circle (Evans et al., GCN 9205) or the NIR position (Tanvir et al., GCN 9202) in the co-added image. We estimate a limiting magnitude (3-sigma) of: z > 26.0 mag (t_mid = 16.25 hr) This observation is nearly co-eval with the GROND photometry (Olivares et al., GCN 9215) which detects the counterpart with a magnitude of J_AB = 21.6, implying a color of J-z > 4.4 (AB). This, along with the non-detection of a lower-redshift host galaxy candidate, the TNG spectrum (Thoene at al., GCN 9216), and the nearly flat JHK SED, confirms the very high redshift (z > 7) of this event. We thank Eleazar Carrasco and the Gemini observing staff for acquiring these observations.