GCN Circular 29307

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
Not found
 
 
) 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
Not found
 
 
) 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.