TITLE: GCN CIRCULAR NUMBER: 9261 SUBJECT: GRB 090423: GROND second epoch imaging and refined SED DATE: 09/04/26 18:38:40 GMT FROM: Thomas Kruehler at MPE/MPI T. Kruehler, J. Greiner and F. Olivares (all MPE Garching) report on behalf of the GROND team: We obtained a second epoch of imaging of the field of GRB 090423 (Swift trigger 350184, H. Krimm et al., GCN #9198), which started on 25 Apr. 2009 at 00:44 UT, 40.8 h after the trigger. In the second epoch, the NIR afterglow (Tanvir et al. GCN #9202) is undetected in stacked images with a total integration time of 4800~s in each of J, H and K down to limiting AB magnitudes of J_AB > 22.7, H_AB > 22.3 and K_AB > 21.6. Together with the magnitudes reported in Olivares et al. (GCN #9215) this implies a fading with a power law index faster than 1.2. This lower limit on the temporal decay index is consistent with that found for the X-ray afterglow at a similar time interval (G. Stratta & M. Perri, GCN #9212). In addition we report on a refined analysis of the GROND photo-z from Olivares et al. (GCN #9215). Using an improved photometric calibration and including the deep z' band upper limit from Perley et al. (GCN #9217) which is contemporaneous to our GROND imaging, we find a refined photometric redshift of 8.0+0.4-0.8 (90% confidence level). The error range is unlikely to improve due to the gap between the z' and J bands (for the negative error), and the width of the J filter band (for the positive error). This range covers the previous claims from photometric (GCNs #9213, #9215, #9217) and spectroscopic observations (GCNs #9219, #9222). The SED is well described with a power law of index ~1. No signatures of intrinsic dust are evident with an upper limit of A_V < 0.5, assuming a SMC dust reddening template.