TITLE:   GCN CIRCULAR
NUMBER:  46147
SUBJECT: GRB 261029A : MITSuME Akeno optical afterglow detection
DATE:    26/11/04 11:39:17 GMT
FROM:    kubo@hp.phys.titech.ac.jp

Y. Kubo, I. Takahashi, M. Sasada, A. Ochi, H. Hagio, R. Kato, N. Hosoya, J. Yonamine, Y. Yatsu and N. Kawai (Science Tokyo) report on behalf of the MITSuME collaboration:

We observed the field of GRB 261029A detected by Fermi (Fermi GBM Team, GCN 46071) and SVOM (Goto et al., GCN 46072) with the optical three-color (g', Rc, and Ic) CCD cameras attached to the MITSuME 50-cm telescope Akeno.
The observations started at 2026-10-29 08:21:36​​ UT (6.9 hours after the trigger). Due to poor and variable weather conditions, we stacked only the images obtained under relatively favorable conditions. We detected an optical afterglow at the position reported by GOTO (O'Neill et al., GCN 46073). The source was subsequently observed by​​ He et al. GCN 46078; Wang et al. GCN 46079; De et al. GCN 46080; Cotter et al. GCN 46081;  Álvarez et al. GCN 46083; Yin et al. GCN 46084; Shi et al. GCN 46088; Kang et al. GCN 46090; Ma et al. GCN 46092; De et al. GCN 46093; Novotny et al. GCN 46097; Zhao et al. GCN 46099; Dimple et al. GCN 46101; Novotny et al. GCN 46102; Rodríguez-Torres et al. GCN 46103; Hua et al. GCN 46104; Yin et al. GCN 46109; Kang et al. GCN 46113; Moskvitin et al. GCN 46133; Calapai et al. GCN 46139; Cheng et al. GCN 46141.

The preliminary photometry of the source is as follows:

T0+[hour] | Band | UT | T-EXP[sec] | Magnitude / 5-sigma limiting magnitude
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11.40 | g’​​| 2026-10-29 12:50:50​​ | 9000 | > 19.4
11.19 | Rc | 2026-10-29 12:38:25​​ | 9840 | 19.6 +/- 0.2
11.19 | Ic | 2026-10-29 12:38:25​​ | 9840 | > 19.1
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T0+ : Elapsed time after the trigger (T0 = 2026-10-29 01:27:03 UT)  
T-EXP: Total exposure time  

We used the PS1 catalog for flux calibration. The catalog magnitudes in PS1 g, r and i band 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, Vol.73, Issue 1, Pages 4-24; https://github.com/MNiwano/Eclaire).