Yuhan Yao (Caltech), Adam Miller (Northwestern), Anna Ho (UCB), Daniel Perley (LJMU)
We observed ZTF20abbiixp/AT2020kym (Ho et al., GCN #27799), the afterglow of the long-duration GRB 200524A (Pookalil et al., GCN #27809; Fana Dirirsa et al., GCN #27797; Gupta et al., GCN #27818; Svinkin et al., GCN #27867), with GMOS-N under our ToO program GN-2020A-Q-117 (PI: Miller). The observation, conducted in the Nod-and-Shuffle mode with a 1 arcsec slit, started at 2020-05-25 07:39:14.9 UT, corresponding to 26.6 hours after the Fermi-GBM trigger (Pookalil et al., GCN #27809). A power-law fit to the afterglow light curve (Ho et al., GCN #27799; Kumar et al., GCN #27800, #27804; Perley & Ho, GCN #27805) suggests that ZTF20abbiixp was at r=21.5 and g=21.8 at the time of Gemini observation.
We obtained 3 x 550 s spectroscopic exposures with the B600 grating and 3 x 550 s exposures with the R400 grating, providing coverage over the range 3620-9600 AA. No flux calibration was performed. The spectrum was reduced using the IRAF package for GMOS. A trace was detected redwards of 4900 AA. Bluewards of 4900 AA, the source was not significantly detected, probably because of the faintness of the object, the intrinsic red color of the afterglow (g-r ~ 0.3 mag) and low sensitivity at the blue end.
We clearly identified absorption lines of Mg II 2796 and Mg II 2803 at the redshift of z = 1.256 in the three individual exposures. Absorption from Mg I 2852 is marginally detected in the combined spectrum at consistent redshift. We therefore identify z=1.256 as the most probable redshift for the GRB, although in practice this is only a lower limit. The lack of a DLA or Lyman break places an upper limit of z < 3.