TITLE: GCN CIRCULAR NUMBER: 39418 SUBJECT: GRB 250221A: VLT/X-shooter redshift z = 0.768 DATE: 25/02/22 04:24:41 GMT FROM: J. T. Palmerio at CEA-Saclay jesse.palmerio@obspm.fr
J. T. Palmerio (CEA/Irfu), A. Saccardi (CEA/Irfu), B. Rayson (U. Leicester), D. B. Malesani (DAWN/NBI and Radboud Univ.), A. J. Levan (Radboud and Warwick), N. R. Tanvir (U. Leicester) report on behalf of the Stargate collaboration:
We observed the optical afterglow (Francile et al., GCN 39395; Caputo et al., GCN 39396; Watson et al., GCN 39397; Melandri et al., GCN 39406; Shilling et al., GCN 39409; Guo et al., GCN 39412; Cotter et al. GCN 39413) of GRB 250221A (Caputo et al., GCN 39396) using the ESO VLT UT3 (Melipal) equipped with the X-shooter spectrograph. Our spectra cover the wavelength range 3000-25000 AA, and consist of 4x600 s exposures. The slit’s position angle was set to cover the afterglow position as well as the nearby object to the NW (Watson et al., GCN 39397; Cotter et al. GCN 39413). The observation mid time was 2025 Feb 22.08 UT (~22.5 hr after the GRB).
In a preliminary reduction of the spectra, we detect two blended traces, corresponding to the afterglow and to the NW object. Spatially consistent with the afterglow position, we detect emission lines of H alpha, [O III] 5007, [O III] 4959, H beta, [O II] 3726/3729 at a common redshift of z = 0.768.
In the UBV arm, the trace is dominated by the afterglow continuum. Absorption features are detected corresponding to Mg II and Fe II, at a consistent redshift, which we suggest is the redshift of the burst.
We should note the presence of a possible, weak emission feature at 16,588 AA at the position of the afterglow trace. We are not able to identify this feature presently and a full reduction of the data is required to ascertain its reality.
At the position of the NW object, a faint, red continuum is detected in the VIS and NIR arms. No clear emission or absorption features are detected, and at the moment we can not provide a redshift determination for this object.
We thank Alan Watson (UNAM) for providing us with accurate coordinates of the afterglow. We acknowledge expert support from the ESO staff in Paranal, in particular Francesca Lucertini and Rodrigo Palominos.
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