TITLE: GCN CIRCULAR NUMBER: 38535 SUBJECT: GRB 241209A: Redshift of the likely host galaxy z = 0.784 from OSIRIS+/GTC DATE: 24/12/11 13:22:54 GMT FROM: Antonio de Ugarte Postigo at LAM/OCA, CNRS deugarte@oca.eu
A. de Ugarte Postigo (OCA and LAM, CNRS), D. B. Malesani (DAWN/NBI and Radboud Univ.), J. F. Agui Fernandez (CAHA), N. R. Tanvir (U. Leicester), C. C. Thoene (ASU-CAS, AbAO), N. A. Rakotondrainibe (LAM), L. Izzo (INAF-OACn & DARK/NBI), A. Martin-Carrillo (UCD), J. P. U. Fynbo (DAWN/NBI), S. Geier (GTC), G. L. Lombardi (GTC), N. Castro Rodriguez (GTC), A. Marante-Barreto (GTC), A. Cabrera Lavers (GTC) report:
The refined X-ray counterpart error region (Evans et al., GCN 38486) of the short GRB 241209A (Fermi collaboration, GCN 38474; Parson et al., GCN 38476; Cie et al., GCN 38478) includes two galaxies, which were identified in FORS2/VLT imaging (de Ugarte Postigo et al., GCN 38484). The fainter one, which was favoured by the initial X-ray localisation (Page et al., GCN 38479) has a redshift of z=1.490, as measured with X-shooter/VLT (Saccardi et al., GCN 38487). The inclusion of the brighter one within the refined X-ray error box, makes it a more likely candidate host for the GRB, because of a lower chance coincidence, as already mentioned by Saccardi et al. The candidate NIR counterpart reported by Schneider et al. (GCN 38506) also lies within the extension of the brighter galaxy. Should it be confirmed to fade, this would also favour this as the GRB host.
We obtained spectroscopy of this, brighter and more likely host galaxy candidate (located at RA = 10:21:35.09, Dec = +06:19:45.4) using OSIRIS+ mounted on the 10.4m GTC telescope, at the Roque de los Muchachos observatory, on the island of La Palma (Spain). The observation consisted of 3x900s using grism R1000R, which has a spectral coverage between 5100 and 10000Å at a resolving power of 670.
The spectrum shows continuum over the complete range and bright emission features that we identify as due to [OII], H-beta and [OIII] as well as CaII H and K absorptions at a common redshift of z = 0.784, which we identify as the redshift of the most likely host galaxy of GRB 241209A. We note that this is consistent with the Legacy Survey photometric redshift of 0.78+/- 0.08 (Zhou et al. 2021, MNRAS, 501, 3309).
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