TITLE: GCN CIRCULAR NUMBER: 41106 SUBJECT: GRB 250716A: Lesedi Detection of Fading Optical Transient GOTO25eyp DATE: 25/07/17 11:01:22 GMT FROM: Amit Kumar at Royal Holloway - UoL/ U of Warwick, UK amitkundu515@gmail.com

Amit Kumar (RHUL), Nikita Rawat (SAAO), David Buckley (SAAO), Justyn R. Maund (RHUL), Danny Steeghs (Warwick) and Raya Dastidar (UNAB) report:

We observed the optical transient GOTO25eyp (AT 2025rhc; Rayson et al., GCN 41103), a candidate counterpart to GRB 250716A (Fermi GBM Team, GCNs 41099, 41102), using the 1-m Lesedi telescope at the South African Astronomical Observatory (SAAO), Sutherland, South Africa. Observations were conducted on 2025 July 17 between 03:08:39 and 04:09:30 UT (~25.66 to ~26.67 hours post-trigger) in g’, r’, i’, and z’ bands (3×360s in g and z, 3×300s in r and i bands).

GOTO25eyp was clearly detected in all individual exposures across the four filters. Preliminary aperture photometry yields the following magnitudes in g’ and r’ bands:

DATE-OBS T-T0 (hrs) Filter Exp (s) Magnitude (AB) ------------------------------------------------------------------------------------- 2025-07-17T03:08:39 25.66 g’ 360s 18.95 ± 0.09 2025-07-17T03:26:56 25.96 r’ 300s 18.67 ± 0.07

These observations indicate a decline of ~0.6 mag over ~8.4 hours since the initial detection reported by GOTO. This fading behaviour suggests a transient origin for GOTO25eyp and is broadly consistent with the temporal evolution expected from a GRB afterglow. We note that we are considering the GOTO L-band (400–700 nm), which approximates the mean wavelength coverage of the g and r filters.

Photometric calibration was performed using reference stars from the SDSS catalogue. Follow-up observations are ongoing, and further analysis is in progress.

Data were obtained using the 1-m Lesedi robotic telescope at the SAAO, equipped with the Mookodi low-resolution spectrograph and imager operating in fully robotic imaging mode. We thank the SAAO Instrumentation and Operations (IO) team for their continued support.

View this GCN Circular online at https://gcn.nasa.gov/circulars/41106. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...