GCN Circular 16169

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
Not found
 
 
) 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
Not found
 
 
) 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
Not found
 
 
;
���from ���
+1.4 hr to +15 hr), and alpha~-1.2 relative to the z-band photometry
published by Maehara et al. (GCN 16151
Not found
 
 
;
���from ���
+2hr to +15.8 hr) and the R-band photometry published by Pandey et al. (GCN
16164
Not found
 
 
;
���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
Not found
 
 
), who find alpha~-1.7 up to
about 6000 s.