TITLE: GCN CIRCULAR NUMBER: 29307 SUBJECT: ZTF21aaeyldq: GTC Redshift and GRB afterglow nature confirmation DATE: 21/01/17 02:18:08 GMT FROM: Alexander Kann at IAA-CSIC A. de Ugarte Postigo (HETH/IAA-CSIC, DARK/NBI), D. A. Kann (HETH/IAA-CSIC), D. A. Perley (LJMU), C. C. Thoene (HETH), M. Blazek, J. F. Agui Fernandez (both HETH/IAA-CSIC), and R. Scarpa (GTC) report: We observed the fast-fading hostless transient ZTF21aaeyldq (Ho et al., GCN #29305) with OSIRIS at the 10.4m Gran Telescopio Canarias (Roque de los Muchachos Observatory, La Palma, Canary Islands, Spain), starting at 2021-01-16 23:35:54.359 UT (16.60 hours after the first optical detection). The source is clearly detected. We obtained a 60 s r'-band finder image and 4 x 900 s spectroscopy with the R1000B grism. For the image starting at 2021-01-16 23:27:09.671 (0.68604 days after the first optical detection), we measure r' = 21.64 +/- 0.03 (AB mag) vs. a nearby SDSS star. The spectrum shows a clear continuum over the full spectral range with many strong absorption lines. We identify these lines as Ly-alpha, SII, OI, SiII, SiIV, CII, CIV, FeII, AlII, and AlIII, at a mean redshift of z = 2.514. We identify this as the redshift of the transient. Coupled with the rapid decay, the moderately red color (Ho et al., GCN #29305) and the lack of any broad spectral features, we also identify this transient as a GRB afterglow for which no high-energy trigger has been reported yet.