GCN Circular 34640

E. Maiorano (INAF-OAS), V. D'Elia (SSDC & INAF-OAR), D. B. Malesani (Radboud Univ. and DAWN/NBI), A. Rossi, E. Palazzi (INAF-OAS), P. D’Avanzo (INAF-OAB), A. Melandri ( INAF-OAR), M. De Pasquale (Messina Univ.) on behalf of CIBO collaboration.

We report the results of the spectroscopic observations of two host galaxy candidates for the short GRB 230205A (Ambrosi et al., GCN 33271
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), first identified by Tomasella et al. (GCN 33302
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) and Urata et al. (GCN 33307
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) in deep Subaru archival imaging.

The optical spectra of the two candidate host galaxies were obtained with the Multi-Object Double Spectrographs (MODS) instrument mounted on the 2x8.4-m LBT telescope (Mt. Graham, AZ, USA) on 2023-03-04, 27 days after the burst trigger. The spectra cover the wavelength range 3200-10000 AA, with a total of exposure time of 1.5 hours.

We detect a faint and red continuum with few emission lines for the brightest galaxy that we tentatively identify as H-alpha, and H-beta at a common redshift of z=0.429 (consistent with the photometric redshift of z = 0.484 ± 0.090 provided by SDSS, see Tomasella et al., GCN 33302
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). The spectral trace of the fainter galaxy is too weak to allow a reliable identification of any emission line, thus not permitting its redshift estimate.

Multiband photometry was also collected between June and July (4-5 months after the GRB trigger) in the r, z and H bands, with both galaxies detected in all filters. These images also show an additional fainter host galaxy candidate at coordinates 13:28:16.56 +46:43:34.7, underlining the complexity of the field.

We acknowledge the excellent support from the LBTO and LBT-INAF staff, particularly D. Thompson, E. Marini, and D. Paris, in obtaining these observations.