GCN Circular 41106
Subject
GRB 250716A: Lesedi Detection of Fading Optical Transient GOTO25eyp
Event
Date
2025-07-17T11:01:22Z (a year ago)
From
Amit Kumar at Royal Holloway - UoL/ U of Warwick, UK <amitkundu515@gmail.com>
Via
Web form
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.