TITLE:   GCN CIRCULAR
NUMBER:  40597
SUBJECT: GRB 250521C/AT2025mgj: FTW optical and NIR observations
DATE:    25/06/01 18:58:47 GMT
FROM:    Malte Busmann at Ludwig-Maximilians-Universität München <m.busmann@physik.lmu.de>

Malte Busmann (LMU), Brendan O’Connor (Carnegie Mellon U.), Daniel Gruen (LMU), and Antonella Palmese (Carnegie Mellon U.) report:

We observed the candidate counterpart AT2025mgj (Andreoni et al., GCN 40566; Martin-Carrillo et al., GCN 40584; Becerra et al., GCN 40590) of GRB 250521C (Fermi GBM team, GCN 40515) with the Three Channel Imager (3KK) at the Fraunhofer Telescope at Wendelstein Observatory (FTW) in the r, i and J band simultaneously for 10 x 180 s starting at 2025-05-30T20:31:19 UT (9.3 days after the GRB trigger). We detect the source at

r = (19.28 +/- 0.01) mag
i = (19.45 +/- 0.02) mag
J = (20.03 +/- 0.07) mag.

We note that the blue color is inconsistent with an afterglow origin, despite the similar power law temporal decay (Martin-Carrillo et al., GCN 40584; Becerra et al., GCN 40590). The source is therefore unlikely to be associated to GRB 250521C. The most likely origin in a Cataclysmic Variable (CV) as noted by Andreoni et al. (GCN 40566). However, given the coincidence with GRB 250521C, we encourage additional observations to conclusively determine the nature of this transient.

The r and i 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 staff for obtaining these observations.