TITLE: GCN CIRCULAR NUMBER: 42329 SUBJECT: GRB 251017A : MITSuME Akeno optical afterglow candidate detection DATE: 25/10/17 16:06:04 GMT FROM: Mahito Sasada at Tokyo Institute of Technology sasada@hp.phys.titech.ac.jp
M. Sasada, A. Ochi, R. Kato, Y. Kubo, H. Hagio, H. Seki, S. Joshima, I. Takahashi, Y. Yatsu and N. Kawai (Science Tokyo) report on behalf of the MITSuME collaboration:
We observed the field of GRB 251017A detected by Swift/BAT (Gupta et al. GCN 42322) with the optical three-color (g', Rc, and Ic) CCD cameras attached to the MITSuME 50-cm telescope Akeno.
The observation started at 2025-10-17 08:41:53 UT (69 seconds after the Swift/BAT trigger). We stacked the images with good conditions. We detected a point source in the Ic-band image at a position of the reported optical candidate (Gupta et al. GCN 42322; Perez-Garcia et al. GCN 42323; Antier et al. GCN 42325). Here we report the magnitude of the source as follows.
T0+[sec] | MID-UT | T-EXP[sec] | magnitude ------------------------------------------------------------------------------------------------ 9256 | 2025-10-17 11:15:00 | 11520 | Ic=19.7+/-0.2 ------------------------------------------------------------------------------------------------ T0+ : Elapsed time after the trigger T-EXP: Total Exposure time
We used the PS1 catalog for flux calibration. The catalog magnitudes in PS1 g, r and i bands were converted to our g', Rc and Ic band magnitudes following Tonry et al. (2012), Table 6. The magnitudes are expressed in the AB system. The images were processed in real-time through the MITSuME GPU reduction pipeline (Niwano et al. 2021, PASJ; https://github.com/MNiwano/Eclaire).
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