TITLE: GCN CIRCULAR NUMBER: 40598 SUBJECT: GRB 250530A: VLT/HAWK-I NIR counterpart fading DATE: 25/06/01 19:26:39 GMT FROM: Ben Rayson at University of Leicester br155@leicester.ac.uk
B. C. Rayson (Leicester), B. Schneider (LAM), P. D'Avanzo (INAF-OAB), A. Saccardi (CEA/Irfu), G. Corcoran (UCD), B. P. Gompertz (U. Birmingham), N. Habeeb (Leicester), D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), G. Pugliese (API), N. R. Tanvir (Leicester) report on behalf of the Stargate collaboration:
We observed the near-infrared source reported by D'Avanzo et al. (GCN 40588), a candidate afterglow of GRB 250530A detected by SVOM/ECLAIRs (Wang et al., GCN 40576). We used the ESO VLT UT4 (Yepun) equipped with the HAWK-I near-infrared camera. We obtained a 20 min exposure in the J band, starting at 23:17:32 UT on 2025-05-31, i.e. 1.7 days after the SVOM trigger.
The near-infrared source is well detected in our image and we measured J = 21.65 +/- 0.10 mag (Vega), calibrated against the 2MASS catalogue. Compared with the value reported by D'Avanzo et al. (GCN 40588), our measurement indicates fading of the counterpart with a power-law decay index of ~0.6. This confirms the source as the NIR afterglow of GRB 250530A.
As noted by D'Avanzo et al. (GCN 40588), this source is consistent with a catalogued source reported in the Legacy Survey. This is the likely GRB host, therefore, the red colors and the optical faintness (Li et al., GCN 40582) are likely due to dust extinction rather than high redshift - as also confirmed by the large column density measured in the X-ray spectrum (Evans et al., GCN 40579; see also: https://www.swift.ac.uk/SVOM/SVOM_FIELD00017/Source1.php).
We acknowledge excellent support from the ESO observing staff in Paranal, in particular Thallis Pessi, Israel Blanchard, Miguel Lopez & Aaron Labdon
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