GCN Circular 16881
V. D'Elia (INAF/OAR & ASI/ASDC), S. Covino (INAF/OAB), M. Cecconi and Carmen Padilla (INAF/TNG) report on behalf of the CIBO collaboration: We observed the optical afterglow of GRB 141004A (D'Elia et al., GCN 16878; Mereghetti et al. GCN 16879) on 2014 October 5 with the 3.6m TNG telescope located in the Canary Islands, equipped with the DOLORES operated in both imaging and spectroscopic mode. The observations were carried out under good weather conditions (seeing about 1.5) at high airmass. In a 180 s image started at 01:41 UT (~2.3 hr after the GRB), the afterglow has a magnitude r = 20.9 +/- 0.1 (AB), calibrated against the APASS catalog. No correction has been applied for the Galactic extinction. The position of the afterglow is RA: 05:06:56.17 Dec: 12:49:10.54, consistent with the UVOT detection (D'Elia et al. GCN 16878) and with the CAHA observation (Gorosabel et al., GCN 16880). We also detect a fainter object (possibly extended) 5" eastward to the afterglow. We also obtained two optical spectra, each one lasting 1200 s, using the grism LR-B covering the wavelength range 3000 - 8000 AA. The observations started at 01:00 UT (~1.7 hr after the GRB). Using a preliminary wavelength calibration, in the co-added spectrum we detect a faint continuum extending down to 4000 A. No obvious absorption or emission features are detected in our preliminary analysis of the afterglow spectrum.