TITLE:   GCN CIRCULAR
NUMBER:  46092
SUBJECT: GRB 261029A: SVOM/VT optical observation
DATE:    26/10/29 14:15:17 GMT
FROM:    Yinuo Ma <mayn@bao.ac.cn>

Y. N. Ma, H. L. Li, L. P. Xin, Y. L. Qiu, C. Wu, Z. H. Yao, J. R. Xu, L. J. Chen, X. H. Han, Y. Xu, J. Wang, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA), Y. F. Liang (PMO), Z. Y. Liu (GZNU) report on behalf of the SVOM/VT team. 

SVOM/VT performed an automatic slew on the burst GRB 261029A (sb26102901) triggered by SVOM/ECLAIRs and SVOM/GRM (Goto et al., GCN 46072). This burst was also detected by Fermi/GBM (Fermi GBM Team, GCN 46071), Fermi/LAT (Longo et al., GCN 46085) and AstroSat CZTI (Salunke et al., GCN 46089). SVOM/VT began observing the field at 2026-10-29T02:12:58 UTC, 45.859 minutes after T0, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously. 

With X-band data available, the optical counterpart (O'Neill et al. GCN 46073; 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) within EP/FXT error box (Kan et al., GCN 46086) was detected in both VT_B and VT_R bands. The AB magnitudes are:

mid time (h) | exposure time (s) | band |  mag (AB) 
-------------|-------------------|------|---------------
     0.77    |         50        | VT_B |  16.21+/-0.01    
     0.77    |         50        | VT_R |  15.41+/-0.01    
     7.94    |         100       | VT_B |  19.93+/-0.04
     7.94    |         100       | VT_R |  19.11+/-0.03

Our photometry was not corrected for Galactic extinction.

The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. VT was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC), CAS.