TITLE:   GCN CIRCULAR
NUMBER:  40859
SUBJECT: GRB 250628A: VLT/X-shooter upper limits
DATE:    25/06/28 14:12:36 GMT
FROM:    Riccardo Brivio at INAF-OAB <riccardo.brivio@inaf.it>

R. Brivio (INAF-OABr), A. Martin-Carrillo (UCD), G. Corcoran (UCD), L. Izzo (INAF/OAC), A. Rossi (INAF/OAS), D. B. Malesani (DAWN/NBI and Radboud), N. R. Tanvir (Leicester) report on behalf of the Stargate collaboration:

We observed the field of GRB 250628A detected by Swift/BAT (Ferro et al., GCN 40853) using the ESO VLT UT3 (Melipal) equipped with the X-shooter acquisition camera.

A short sequence of imaging was secured in the r (3x30 s), g (3x40 s), and z (3x60 s) bands. No afterglow is detected within the Swift/XRT enhanced error region (Evans et al., GCN 40854).
From preliminary analysis, we measure the following 3sigma upper limits, calibrated against nearby stars from the Legacy Survey (DR9):

g > 23.7 (AB) on Jun 28 at 08:51:48 UT (7.93 hr after the GRB trigger),
r > 23.6 (AB) on Jun 28 at 08:45:04 UT (7.82 hr after the GRB trigger),
z > 22.8 (AB) on Jun 28 at 08:48:21 UT (7.87 hr after the GRB trigger).

We note the detection of significant excess absorption in the X-ray spectrum (Evans, GCN 40857; https://www.swift.ac.uk/xrt_spectra/01328546/). This argues against a high-redshift origin for GRB 250628A and likely indicates some dust extinction.

We acknowledge excellent support from the ESO observing staff in Paranal, in particular Boris Häußler.