A. de Ugarte Postigo (IAA-CSIC, DARK/NBI), C.C. Thoene (IAA-CSIC),
J.P.U. Fynbo (DARK/NBI), N.R. Tanvir (U. Leicester), R. Sanchez-Ramirez
(IAA-CSIC) and S. Geier (GRANTECAN), report on behalf of a larger collaboration:
We observed the afterglow of GRB 160104A (Melandri et al. GCN18815,
Guidorzi et al. GCN18816) with OSIRIS on the 10.4 m GTC telescope, located
at the Roque de los Muchachos Observatory (La Palma, Spain). Observations
started at 20:25 UT (9.0 hr after the GRB onset) and consisted of a 40 s
acquisition image in i-band as well as 3x900s spectra using the R1000R grism which
covers the range between 5100 and 10000 AA with a resolution of R~800.
The acquisition image was obtained under good conditions with a seeing of 0.8".
The afterglow is clearly detected although it shows a strong decay in brightness
with respect to previous detections (Guidorzi et al. GCN18816, Saito et al. GCN
18817, Yurkov et al. GCN 18820, Xin et al. 18823). As compared to USNO-B1.0
stars we measure a magnitude of i(Vega)=22.6, roughly equivalent to i(AB)=23.0.
The combined spectrum shows a clear continuum over the complete range
with only one prominent absorption feature centred at 5889 AA, with an
equivalent width of ~30 AA. There are also tentative absorptions features
at ~5300 and ~5340 AA. There is no drop bluewards of the 5889 AA feature
which could otherwise be suggestive of a weak Ly-alpha at z=3.84. At a
redshift of 2.80, the features could be consistent with CIV and the two SiIV,
respectively. We are not able to establish a convincing solution for the redshift
based on this evidence.