TITLE:   GCN CIRCULAR
NUMBER:  40846
SUBJECT: GRB 250625A: GTC/OSIRIS+ spectroscopic observations of the potential host galaxy
DATE:    25/06/26 23:08:02 GMT
FROM:    Antonio de Ugarte Postigo at LAM, CNRS <adeugartepostigo@gmail.com>

A. de Ugarte Postigo (LAM), A. Martin-Carrillo (UCD), D. B. Malesani (DAWN/NBI and Radboud), J. F. Agui Fernandez (CAHA), M. A. Aloy (UV), J. P. U. Fynbo (DAWN/NBI), L. Galbany (IEEC-CSIC), S. Geier (GTC), B. Schneider (LAM), L. Izzo (INAF/OACN and DARK/NBI), G. Lombardi (GTC), N. A. Rakotondrainibe (LAM), N. R. Tanvir (Univ. Leicester), C. C. Thoene (AbAO), F. Pérez (GTC), D. Pérez Valladares (GTC) and A. Cabrera Lavers (GTC) report:

We observed the optical afterglow of the GRB 250607A detected by Fermi/GBM and Swift/BAT (Fermi GBM team, GCN 40824; Ferro et al., GCN 40825) using the 10.4 m GTC located at the Roque de los Muchachos Observatory in the island of La Palma (Spain) equipped with the OSIRIS+ instrument. 

In a 3x30 s acquisition image in the z-band, with a mean time of 2025-06-26T01:09:07 (~9.07 hr after the Swift trigger), we detect two objects consistently with the Swift/XRT position (REF). The first is the optical afterglow reported by Moskvitin et al. (GCN 40828), Xin et al. (GCN 40830), Zhu et al. (GCN 40834), Fortin et al. (GCN 40837), Adami et al. (GCN 40838) and Brosio et al. (GCN 40839), for which we measure a magnitude z = 23.75 +/- 0.20 (AB, calibrated against nearby Legacy Survey objects). The second (at RA =17:26:02.17, Dec = +22:16:02.8) has an AB magnitude z = 23.69 +/- 0.18, with a separation of ~1.4" from the afterglow.

Our spectroscopic observations started on 2025-06-26T01:44:27.5 UT (9.67 hr after the Swift trigger) and consisted of 3x1200 s exposures using grism R1000R, covering the range between 5100 and 10,000 AA. 

In the acquisition process, the slit was placed on top of this second object, not the afterglow. For this target, in a preliminary reduction of the spectra, we detect a continuum and a few emission lines, including the [O II] doublet, [O III] 5008 and H-beta, from which we infer a redshift of z = 0.655. At this distance, if this galaxy is associated with the optical afterglow, the offset between the galaxy and the optical afterglow would be ~10 kpc.

We note that the probability of chance coincidence is not negligible (~2.5%). We cannot thus conclude that GRB 250625A is at redshift z = 0.655. Further observations to confirm their association are planned and encouraged.

We thank Liping Xin (NAOC) for sharing accurate coordinates of the afterglow from SVOM/VT.