TITLE: GCN CIRCULAR NUMBER: 41431 SUBJECT: GRB 250818B: optical afterglow detection with Lesedi (GOTO25fzq) DATE: 25/08/19 14:54:41 GMT FROM: Amit Kumar at Royal Holloway - UoL/ U of Warwick, UK amitkundu515@gmail.com
A. Kumar (RHUL), D. O’Neill (Birmingham), N. Rawat (SAAO), R. A. J. Eyles-Ferris (U. Leicester), R. Dastidar (UNAB), D. Buckley (SAAO), D. Steeghs (Warwick), J. R. Maund (RHUL), R. L. C. Starling (U. Leicester) and G. Ramsay (Armagh) report:
We observed the optical afterglow GOTO25fzq (Kumar et al. GCN 41406) associated with GRB 250818B (Wang et al., GCN 41405), also reported by Yao et al. (GCN 41409), Zheng et al. (GCN 41417), Bendtsen et al. (GCN 41426), Moskvitin et al. (GCN 41428), and An et al. (GCN 41430). The observations are conducted using the 1-m Lesedi telescope at the South African Astronomical Observatory (SAAO), Sutherland, South Africa, between 2025-08-19 00:51:24 UT and 2025-08-19 04:05:00 UT (between 21.37 and 24.60 hours post-trigger) in g’, r’, i’, and z’ bands (6×450s in g and z, 6×400s in r and i bands).
OT was clearly detected in the stacked g’- and r’-band images, whereas i’ and z’ images are severely affected by trailing. Preliminary aperture photometry yields the following magnitudes in the g’ and r’ bands:
DATE-OBS (Start_time) T-T0 (hrs) Filter Exp (s) Magnitude (AB) -------------------------------------------------------------------------------------- 2025-08-19 00:51:24 UT 21.37 g’ 6x450s 21.39 ± 0.09 2025-08-19 01:36:43 UT 22.13 r’ 6x400s 21.58 ± 0.47
Although our r'-band image is also significantly affected by trailing, aperture photometry confirms a low-significance detection, yielding the reported magnitude.
Photometric calibration was performed using reference stars from the SDSS catalogue. Magnitudes are not corrected for Galactic extinction. 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.
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