[vsnet-grb-info 14855] GRB 140423A: NOT Afterglow Observations

[GCN Circulars]

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  <zewcano at gmail.com>

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.