TITLE: GCN CIRCULAR NUMBER: 40392 SUBJECT: GRB 250506A: SVOM/VT optical upper limit DATE: 25/05/08 07:22:50 GMT FROM: SVOM_group svomgroup@bao.ac.cn

L. Lan(NAOC), C. W. Wang(IHEP), R. C. Chen(NJU), W. J. Tan(IHEP), L. P. Xin, Y. L. Qiu, H. L. Li, C. Wu, Z. H. Yao, Y. N. Ma, 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. T. Palmerio (CEA) report on behalf of the SVOM mission team.   After the trigger by SVOM/ECLAIRs and Fermi/GBM at 2025-05-06T02:23:22 UTC (SVOM burst-id sb25050601, Fermi burst-id bn250506100), VT performed ToO observations on the burst in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously. The observation started at 13.69 hours after the SVOM trigger.

No uncataloged sources are detected in single or stacked images within the errorbox of Eclairs or MXT (Maggi et al., GCN 40364), compared to the Legacy survey.

The 3 sigma upper limits are derived below:

Mid_time | Band | Exposure Time (second) | Upper limit (AB) 15.64 hour          | VT_B | 100*63  | 23.7 15.64 hour          | VT_R | 100*73  | 23.6   This results are consistent with the report (de Ugarte Postigo et al. GCN 40359).

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.

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