TITLE:   GCN CIRCULAR
NUMBER:  37129
SUBJECT: GRB 240809A: Conclusive redshift from VLT
DATE:    24/08/10 10:16:16 GMT
FROM:    Daniele B. Malesani at IMAPP / Radboud University <d.malesani@astro.ru.nl>

B. Schneider (MIT), D. B. Malesani (DAWN/NBI and Radboud Univ.), A. de Ugarte Postigo (CNRS), J. P. U. Fynbo (DAWN/NBI), R. Gottumukkala (DAWN/NBI), L. Izzo (INAF/OACn and DARK/NBI), N. R. Tanvir (Univ. Leicester), D. Xu (NAOC), report on behalf of the Stargate collaboration:

We observed the afterglow of GRB 240809A (Evans et al., GCN 37110; Wang et al., GCN 37113; Dubay et al., GCN 37114; Jiang et al., GCN 37116; Gottumukkala et al., GCN 37122; Shilling & Evans, GCN 37123; Mohan et al., GCN 37125; Zheng & Filippenko, GCN 37127) using the X-shooter spectrograph mounted on the ESO VLT UT3 (Melipal). The observation mid time was 2024 Aug 10.056 UT (16.84 hr after the GRB), consisted of 6 exposures of 600 s each and covered the wavelength range 3000-21000 AA.

In images taken with the acquisition camera, we clearly detect the optical afterglow. In the r band, we measure an AB magnitude of r ~ 20.9, calibrated against the Pan-STARRS catalog.

In a preliminary reduction, we clearly detect continuum over the entire wavelength range. From detection of multiple absorption features, which we interpret as due to Fe II, Mg II, C IV, Al lI and Si II, we infer a common redshift of z = 1.494. No emission lines from the underlying host galaxy are detected in a preliminary analysis. This redshift is perfectly consistent with the value by Gottumukkala et al. (GCN 37122). We thus confirm that z = 1.494 is the redshift of GRB 240809A. We also note the presence of additional absorption features, likely due to an intervening system at z = 0.844.

We acknowledge expert support from the ESO staff in Paranal, in particular Maria Jose Rain, Rob van Holstein, and Boris Haeussler.