TITLE:   GCN CIRCULAR
NUMBER:  39769
SUBJECT: GRB 250317B: GTC/OSIRIS+ spectroscopic redshift z=3.44
DATE:    25/03/18 12:43:40 GMT
FROM:    Christina Thöne at ASU-CAS <christina.thoene@gmail.com>

C. C. Thoene (AbAO), A. de Ugarte Postigo (LAM), D. B. Malesani (DAWN/NBI and Radboud Univ.), J. F. Agui Fernandez (CAHA), N. A. Rakotondrainibe (LAM), A. Martin-Carrillo (UCD), N. R. Tanvir (Univ. Leicester), L. Izzo (INAF/OACn and DARK/NBI), J. P. U. Fynbo (DAWN/NBI), S. Geier (GTC), G. Lombardi (GTC), Antonio García (GTC), Daniel González (GTC), and Antonio Cabrera-Lavers (GTC) report:

We obtained spectroscopy of the afterglow of the SVOM-detected GRB 250317B (Zhao et al. GCN 39752, Watson et al. GCN 39754, Götz et al. GCN 39756, Pérez-Fournon et al. GCN 39757, Melandri et al. GCN 39763, Ferro et al. GCN 39764, Schneider et al. GCN 39767). Our observations consisted of 4x1200s exposures with grism R1000B, covering the range between 3600 and 7880 AA with a resolving power of around 600 at a mean epoch 2025-03-18T00:16:22 UT (22.07 hr after the burst).

In the acquisition image, obtained 21.328 hr after the burst, the afterglow is detected at a magnitude of r_AB=21.72 +/- 0.05, indicating a slight rebrightening with respect to the OHP observation (Schneider et al. GCN 39767) obtained 1.6 hr earlier. Further monitoring of this behaviour is encouraged.

The spectrum shows continuum above 4000 AA, with features consistent with at least two low-column density Lyman systems at redshifts of z=3.442 and 3.447 (or a dynamic separation of 1500 km/s), likely corresponding to the same galaxy or system. There are tentative detections of SiII, SiII*, OI, SiIV, CIV, FeII and AlII a those same redshifts, which we identify as corresponding to the GRB.

Additionally, we identify two intervening absorbers, one at z=3.196 with absorptions in several Lyman lines, SII and SiII, and a second one at z=1.029 showing the MgII doublet.