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.
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