GCN Circular 18177

R. Sanchez-Ramirez (UPV/EHU, IAA-CSIC), J. Gorosabel (+, UPV/EHU,
IAA-CSIC), D. Perez-Ramirez (+, Univ. de Jaen), S. Jeong (IAA-CSIC, SKKU),
A. J. Castro-Tirado (IAA-CSIC, ISA-UMA), F. J. Aceituno, R. Cunniffe, P.
Ferrero, Y. Hu, S. R. Oates, J. C. Tello, B.-B. Zhang (IAA-CSIC), M.
Jelinek (ASU-CAS), S. Guziy (Nikolaev Univ.), V. Sokolov (SAO-RAS), J.M.
Castro Ceron (ESAC), J. Cepa (IAC), A. Garcia (GRANTECAN) and R. Scarpa
(IAC, GRANTECAN), report:

Following the detection of GRB 150818A by Swift/BAT (D Elia et al. 2015,
GCNC 18152), we have obtained an optical spectrum (2 x 900s) with the
10.4m GTC telescope (+OSIRIS) in La Palma (Spain), starting on Aug 18,
22:05 UT (i.e. 10.5 hr postburst), covering the 3700-7800 A wavelength
range. The slit included the position of the proposed optical afterglow
(Marshall and D Elia, GCNC 18155; Schulze et al., GCNC 18164; Mazaeva et
al. GCNC 18175).

At the position of the optical afterglow, we see no obvious absorption
features with the continuum extending from 3700 to 7800 A. However we
clearly detect emission lines of [OII] 3727A, H-beta and [OIII]
4960A,5008A, all at a common redshift z = 0.282. Indeed a diffuse object
seems to be adjacent (within 1� to the west) to the optical afterglow in
the acquisition image (under 0�.7 seeing), which could be the host galaxy
at the above mentioned redshift.

Further spectroscopy and monitoring at all wavelengths is encouraged to
search for the emergence of the underlying supernova component in the next
weeks.