GCN Circular 15307
R. Chornock, R. Lunnan, and E. Berger (Harvard) report: We observed the optical afterglow (Xu et al., GCN 15304; Malesani et al., GCN 15305; Kopac et al., GCN 15306) of the possible short GRB 131004A (Hagen et al., GCN 15303) using LDSS-3 on the 6.5-m Magellan Clay Telescope. A sequence of 3x1200s spectroscopic exposures of the afterglow began at 23:44 on 4 October UT. A preliminary reduction reveals a flat continuum with superposed weak emission lines from [O II], H-beta, and [O III] at a common redshift of 0.717. Possible weak features consistent with Ca II H+K absorption at this same redshift are also present.