GCN Circular 29945
Yuhan Yao (Caltech), Anna Ho (UC Berkeley), Daniel Perley (LJMU) We observed the candidate afterglow ZTF21aayokph/AT2021lfa (Yao, Perley, and Ho, GCN Circ. 29938) with GMOS-S under our ToO program GS-2021A-Q-124 (PI: Ho). The observation, conducted in the Nod-and-Shuffle mode with a 1 arcsec slit, started at 2021-05-05 04:19:19 UT, corresponding to 22.7 hours after the first ZTF detection. We obtained 2 x 450 s spectroscopic exposures with the B600 grating and 2 x 450 s exposures with the R400 grating, providing coverage over the range 3620-9540 AA. No flux calibration was performed. The spectrum was reduced using the IRAF package for GMOS. We clearly identified absorption lines of Mg II 2796 and Mg II 2803 at the redshift of z = 1.063 in the B600 and R400 spectra. Absorption lines from Mg I and Fe II are marginally detected in the spectra at consistent redshift. We therefore identify z=1.063 as the most probable redshift for AT2021lfa, although in practice this is only a lower limit. The lack of a DLA places an upper limit of z < 2.3. We thank the GMOS-S observing staff for rapidly scheduling and executing our observations, in particular Jeong-Eun Heo.