TITLE: GCN CIRCULAR NUMBER: 16169 SUBJECT: GRB 140423A: NOT Afterglow Observations DATE: 14/04/24 16:49:40 GMT FROM: Zach Cano at U of Iceland Z. Cano (U. Iceland), D. Malesani (DARK), S. Geier (NOT), J. J. Jensen (DARK), F. Taddia, C. Fremling (Stockholm U.) We observed the field of GRB 140423A (Sonbas et al., GCN 16142) with the 2.5-m Nordic Optical Telescope (NOT) equipped with ALFOSC starting at 23:32 UT on 23-Apr-2014. We obtained 2x150 s, 6x200 s and 7x200 s frames in SDSS-r, SDSS-i and SDSS-z respectively. The optical afterglow ( ​e.g. ​ Elenin et al., GCN 16143) is clearly detected in our co ​-​ added images. A summary of our photometry: filter t-t0 (hr) mag --------------------------------------------- r 15.06 21.46 +- 0.03 i 15.33 21.28 +- 0.03 z 15.78 21.36 +- 0.07 Our photometry has been calibrated using an SDSS star in the FOV (13:09:09.56 +49:52:13.38, r=19.76 +- 0.02, i=19.23 +- 0.02, z=18.92 +- 0.04), it has not been corrected for foreground extinction, and the errors include the uncertanity in the SDSS calibration star. Comparing our observations with those in the GCNs, we derive a decay constant of alpha~-1.0 relative to the photometry published by Cenko & Perley (GCN 16153; ​from ​ +1.4 hr to +15 hr), and alpha~-1.2 relative to the z-band photometry published by Maehara et al. (GCN 16151; ​from ​ +2hr to +15.8 hr) and the R-band photometry published by Pandey et al. (GCN 16164; ​from ​ +6 hr to +15 hr), where a colour term of r-Rc~0.19 has been assumed in the latter. These decay rates are slower than those derived from the early KAIT observations (Zheng et al., GCN 16156), who find alpha~-1.7 up to about 6000 s.