[vsnet-grb-info 17803] GRB 160327A: Imaging and spectroscopy from GTC

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  19245
SUBJECT: GRB 160327A: Imaging and spectroscopy from GTC
DATE:    16/03/29 18:18:21 GMT
FROM:    Antonio de Ugarte Postigo at IAA-CSIC  <deugarte at iaa.es>

A. de Ugarte Postigo (IAA-CSIC, DARK/NBI), N.R. Tanvir (U. Leicester), 
Z. Cano (U. Iceland), L. Izzo (U. Roma1), J.P.U. Fynbo (DARK/NBI), 
R. Sanchez-Ramirez (IAA-CSIC), C.C. Thoene (IAA-CSIC), P. Pesev 
(GRANTECAN, IAC, ULL) report:

We observed the field of GRB 160327A (Racusin, et al., GCN 19235) with 
OSIRIS mounted on the 10.4 m GTC telescope, located at Roque de los 
Muchachos Observatory (La Palma, Spain) starting at 00:26 UT on 28th 
March 2016 (15.17 hr after the burst).

In the acquisition image, we identify the afterglow proposed by Golkhou 
et al. (GCN 19236) and later also observed by Zheng & Filippenko (GCN 
19242). At the time of our observation, the object was at a magnitude 
of i(AB) = 23.89 +/-0.12 (as compared to nearby SDSS stars), with 
coordinates of (J2000.0, +/-0.5"):

R.A.: 09:46:48.56
Dec.: +54:00:46.3

We also note the presence of two other sources within 2.5” of the GRB 
afterglow: The first one is located at 09:46:48.35, +54:00:47.8 and has a 
magnitude of i(AB) = 23.76+/-0.11, and the second one is at 09:46:48.55, 
+54:00:44.2 with a magnitude of i(AB) = 24.24+/-0.15. We suspect that 
these sources may be contributing to the magnitudes given by Golkhou 
et al. (GCN 19243), especially in the bluer bands.

Spectroscopy of the GRB afterglow and the first of the two other objects 
was obtained by exposing 3x1200 s with grism R1000R, which covers 
between 5100 and 10100 Angstrom. We detect a trace of the GRB 
afterglow above 7300 AA, below which the emission drops abruptly. The 
other object shows a continuum over the complete spectral range, 
suggesting that this object is unrelated to the GRB. The afterglow 
spectrum is indicative of a Lyman-alpha drop at a redshift between 4.90 
and 5.01, with a most probable value at z = 4.99, although even lower 
redshifts could not be discarded for unusually large Hydrogen column 
densities. Due to the low S/N of the spectrum we are not able to confirm 
any other absorption lines at this time.