TITLE: GCN CIRCULAR NUMBER: 38646 SUBJECT: GRB 241209B: host galaxy detection and GTC/OSIRIS+ spectroscopy DATE: 24/12/21 21:45:12 GMT FROM: Antonio de Ugarte Postigo at LAM/OCA, CNRS <deugarte@oca.eu> A. de Ugarte Postigo (CNRS, OCA and LAM), D. B. Malesani (DAWN/NBI and Radboud Univ.), N. R. Tanvir (Univ. Leicester), L. Izzo (INAF/OACn and DARK/NBI), J. F. Agui Fernandez (CAHA), J. P. U. Fynbo (DAWN/NBI), S. Geier (GTC), G. Lombardi (GTC), A. Martin-Carrillo (UCD), N. A. Rakotondrainibe (LAM), C. C. Thoene (ASU-CAS, AbAO), N. Castro-Rodriguez (GTC), report: At the location of the optical afterglow (Qiu et al., GCN 38516, GCN 38568) of the SVOM GRB 241209B (Xie et al., GCN 38478; see also DeLaunay et al., GCN 38428; Dafcikova et al., GCN 38534; Ridnaia et al., GCN 38537), a faint object is visible in the Legacy Survey r-band image (its detection in the g and i bands is marginal at best). We measure r = 23.52 +/- 0.12 mag (AB) calibrated against nearby Pan-STARRS objects. This is likely the GRB host galaxy. A spectrum of this object was secured using OSIRIS+ on the 10.4 m GTC telescope, at the Roque de los Muchachos Observatory (La Palma, Spain) on the 11th December 2024 (2.06 days after the burst). Blind offsets were used to perform acquisition. The observation consisted of 3 spectra of 1200 s each, using grism R1000R which provides coverage over the spectral range 5100-10200 AA at a resolution R ~ 700. On top of faint continuum, a single emission line is detected at the observed wavelength of 5872 AA. We discuss possible interpretations of this feature. (A) The line is unlikely to be [O III] 5008 at z = 0.173, as the galaxy would be very faint and we see no hint of [O III] 4959 nor Halpha. (B) The line could be the [O II] doublet at z = 0.575. At this redshift, Halpha falls out of the covered range, Hbeta lands in the telluric A band and [O II] 5008 in a region affected by strong sky line residuals. We believe this to be the most favoured solution, but the lack of [OIII] detection, could imply that it is fainter than [O II], which is uncommon for both short and long GRB host galaxies (Kruehler et al. 2014, A&A, 581, A125; Fong et al. 2022, ApJ, 940, 56). (C) The feature could be Ly-alpha at z = 3.831. At this redshift, the galaxy would have an unprecedented luminosity with an absolute UV magnitude M = -22.5 (AB), significantly brighter than other GRB hosts at comparable redshift (e.g., Hjorth et al. 2012, ApJ, 756, 187; Sears et al. 2024, ApJ, 966:133). however, this interpretation would however be consistent with a mild break in the continuum blueward of the line, which could be the onset of the Lyman forest, as well as with the blue color of the VT afterglow (VT R-B = 1.45 +/- 0.20 mag: Qiu et al., GCN 38516). With the available data, it is difficult to provide a secure redshift. Imaging of the host is planned, pending favorable weather, in order to constrain the redshift photometrically and discriminate between the two possibilities. View this GCN Circular online at https://gcn.nasa.gov/circulars/38646. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...