TITLE: GCN CIRCULAR NUMBER: 41445 SUBJECT: GRB 250818B: FTW optical and NIR observations show reddening of the counterpart DATE: 25/08/20 14:06:15 GMT FROM: Malte Busmann at Ludwig-Maximilians-Universität München m.busmann@physik.lmu.de

Malte Busmann (LMU), Jennifer Fabà (LMU), Vani Jain (LMU), Xiaoxiong Zhou (LMU), Daniel Gruen (LMU), Brendan O’Connor (Carnegie Mellon U.), and Antonella Palmese (Carnegie Mellon U.) report:

We observed the counterpart of the short GRB 250818B (Wang et al., GCN 41405; Kumar et al., GCN 41406; Ferro et al., GCN 41407; Yao et al., GCN 41409; Yheng et al., GCN 41417; Fong et al., GCN 41419; Li et al., GCN 41424; Bendtsen et al., GCN 41426; Moskvitin et al., GCN 41428; An et al., GCN 41430; Kumar et al., GCN 41431; Siegel et al., GCN 41435; Dimple et al., GCN 41442) with the Three Channel Imager (3KK) at the Fraunhofer Telescope at Wendelstein Observatory (FTW) in the r, i and J-band simultaneously for 35x 180 s starting at 2025-08-19T01:13:45 UT (0.91 days after the trigger) and again for 30x 180 s starting at 2025-08-20T01:27:02 (1.9 days after the trigger). In the two epochs, we detect the counterpart at

r = (21.12 +/- 0.02) mag (1st epoch), r = (22.26 +/- 0.08) mag (2nd epoch).

We note a significant reddening between the two epochs, where the r-J color increases by nearly 0.6 mag. The cause of the color evolution is unclear, but it is potentially consistent with either a red host galaxy or an intrinsic color evolution of the counterpart. Further observations are encouraged.

The r-band magnitudes are calibrated against the PS1 catalog, and the J-band is calibrated with the 2MASS Catalog. All magnitudes are provided in the AB system and are not corrected for Galactic extinction.

We thank Michael Schmidt from the Wendelstein Observatory for obtaining these observations.

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