TITLE:   GCN CIRCULAR
NUMBER:  32099
SUBJECT: GRB 220521A: Gemini Observations
DATE:    22/05/23 23:35:25 GMT
FROM:    Robert Strausbaugh at U. of the Virgin Islands  <robert.strausbaugh@uvi.edu>

 A. Cucchiara (College of Marin), R. Strausbaugh (U. of the Virgin Islands), J. Rastinejad, A. Nugent, W. Fong (Northwestern), D. B. Malesani (Radboud Univ. and DAWN/NBI) report on behalf of a larger collaboration:

On May 22.16 UT (T0+4.5 hr after the GRB) we observed the afterglow, of GRB 220521A (Fermi GBM team, GCN 32075; Dichiara et al., GCN 32076; Poolakkil & Meegan, GCN 32089; Strausbaugh et al., GCN 32091) with the Gemini-South telescope and the GMOS camera.

We performed a series of spectroscopic observations with the R400 grating centered at 7500 AA (2x600 s) and 8100 AA (2x600 s). Observations were obtained at an average airmass of 2.34.  In our 300 s, z-band acquisition image we detected the GRB counterpart at the NOT optical afterglow location (Malesani et al., GCN 32078).

In our preliminary analysis we measured z (AB)=20.9 +/- 0.3, calibrated against 10 PS1 stars. No extinction correction has been applied.

Our visual inspection of the spectroscopic data confirms the redshift reported by Fynbo et al. (GCN 32079) as z=5.57 based on the detection of Lyman alpha, SiIV and Ni lines.

Combined with the results from MMT (Rastinejad et al., GCN 32097)we derive a decay index in z-band of alpha=1.57, consistent with the current X-ray behavior  (Tohuvavohu et al., GCN 32086). Further analysis is in progress.

We thank the Gemini-S staff, particularly Javier Fuentes and Veronica Firpo, for conducting these observations.