TITLE:   GCN CIRCULAR
NUMBER:  14478
SUBJECT: GRB 130427A: NOT optical photometry and redshift
DATE:    13/04/28 00:47:29 GMT
FROM:    Dong Xu at DARK/NBI  <dong.dark@gmail.com>

D. Xu (DARK/NBI), A. de Ugarte Postigo (IAA-CSIC, DARK/NBI), S.
Schulze (PUC and MCSS), J. Jessen-Hansen (AU), G. Leloudas (OKC,
Stockholm), T. Kruehler, J.P.U. Fynbo (DARK/NBI), P. Jakobsson (U.
Iceland) report on behalf of a larger collaboration:

We observed the field of GRB 130427A (Maselli et al., GCN 14448) with
the 2.5m Nordic Optical Telescope (NOT) equipped with the ALFOSC
camera. Observations started at 20:39:12 UT on 2013-04-27 (i.e., 12.85
hr after the BAT trigger).

We first obtained a series of frames in the sloan u-, g-, r-, i-, and
z-band filters. The optical afterglow (ref., Elenin et al., GCN 14450)
is clearly detected in the stacked images in each of these filters.
Preliminary analysis gives the following magnitudes

u=17.70+/-0.03
g=17.34+/-0.03
r=17.07+/-0.03
i=16.94+/-0.03
z=16.88+/-0.03

at a mean time of 12.96 hr post-trigger, calibrated with nearby stars
in the SDSS field.

We also carried out a 1800s spectroscopic exposure. The spectrum
covers 3200 - 9100 AA at a resolution of ~700 and has a high S/N. We
detect prominent absorption lines of the MgII doublet, MgI and CaII K
& H as well as weak emission lines of the [OII] doublet and H-beta,
all at a common redshift of z=0.338+/-0.002, consistent with the
redshift reported by Levan et al. (GCN 14455).