GCN Circular 20077

A. J. Castro-Tirado (IAA-CSIC), A. F. Valeev (SAO-RAS), V. V. Sokolov (SAO-RAS), P. Ferrero (IAA-CSIC), J. C. Tello (IAA-CSIC), Y. Hu (IAA-CSIC), B.-B. Zhang (IAA-CSIC), S. Jeong (SKKU and IAA-CSIC), J. Cepa (IAC), S. R. Oates (U. of Warwick), D. Garcia-Alvarez (IAC, ULL, GRANTECAN), D. Perez Valladares (GRANTECAN), N. Castro-Rodriguez (GRANTECAN) and S. Fernandez (GRANTECAN) on behalf of a larger collaboration, report:

Following the detection of GRB 161017A by Swift (Troja et al. GCNC 20064), Fermi (Hui and Meegan, GCNC 20068)
and Lomonosov (Sadovnichy et al. GCNC 20075), we observed the optical afterglow (Yurkov et al. GCNC 20063; Breeveld and Troja, GCNC 20074) with the 10.4m Gran Telescopio Canarias (GTC) at the Spanish island of La Palma, starting at 05:59 UT on Oct 18 (i.e. 12.1 hr post-burst), with both the R1000B and R2500I OSIRIS grisms, covering a combined range of 3800-10000 A. The reddest spectrum (convering the range between 7350 and 10000 A at a resolution of 2500) shows the strong Mg II doublet at a redshift z = 2.0127, consistent with the value reported by de Ugarte Postigo et al. (GCNC 20069). We also identify, in the bluest spectrum, other absorption lines (eg. SiII, FeII) and the two intervening systems reported on their GTC (+OSIRIS) R1000B spectrum taken 1.3 hr earlier than us.

We acknowledge excellent support from the GTC staff.