TITLE:   GCN CIRCULAR
NUMBER:  41549
SUBJECT: EP250821a: Gemini GMOS Redshift Confirmation
DATE:    25/08/26 15:49:43 GMT
FROM:    Brendan O'Connor at Carnegie Mellon University <boconno2@andrew.cmu.edu>


Brendan O'Connor (CMU), Xander Hall (CMU), Antonella Palmese (CMU) report on behalf of a larger collaboration:

We observed the location of EP250821a (Hu et al. GCN 41459; Liang et al., GCN 41467) with the Gemini GMOS-S spectrograph starting on 2025-08-26 at 01:53:43 UT. Our spectra cover wavelengths 575 to 1060 nm and consist of 4x1200 s exposures using the R400 grating. In a preliminary reduction, at the location of the optical counterpart (Dichiara et al., GCN 41469; Li et al. GCN 41470; Yao et al. GCN 41485), we clearly detect a bright trace. We detect multiple emission lines from the underlying host galaxy, including [OII], [OIII], Hbeta, [NII], and Halpha, at a consistent redshift of z=0.577. This confirms the previous spectroscopic redshift obtained by the VLT (An et al., GCN 41490).

We thank the staff of the Gemini Observatory, in particular Karleyne Silva, Cinthya Rodriguez, Aleksandar Cikota, Emanuele Farina, for assistance with scheduling and obtaining these observations.