TITLE: GCN CIRCULAR NUMBER: 37581 SUBJECT: EP240918a: SVOM/VT upper limit DATE: 24/09/20 22:22:03 GMT FROM: Liping Xin at NAOC, SVOM xlp@nao.cas.cn
SVOM/VT commissioning team: Y. L.Qiu, L. P. Xin, H. L. Li, C. Wu, X. H. Han, W. J. Xie, H. B. Cai, Y. Xu, Y. J. Xiao, P. P. Zhang, J. S. Deng, L. Lan, X. M. Lu, R. S. Zhang (NAOC), J. Zhang, L. J. Dan, G. Y. Zou, C. J. Wang, Y. F. Du, C. Huang (XIOPM) and Palmerio, J. (CEA).
SVOM JSWG: Jian-Yan Wei (NAOC), Bertrand Cordier (CEA), Shuang-Nan Zhang (IHEP), Stéphane Basa (LAM), Jean-Luc Atteia (IRAP), Arnaud Claret (CEA), Zi-Gao Dai (USTC), Frédéric Daigne (IAP), Jin-Song Deng (NAOC), Andrea Goldwurm (APC), Diego Götz (CEA), Xu-Hui Han (NAOC), Cyril Lachaud (APC), En-Wei Liang (GXU), Yu-Lei Qiu (NAOC), Susanna Vergani (Obs.Paris), Jing Wang (NAOC), Chao Wu (NAOC), Li-Ping Xin (NAOC), Bing Zhang (UNLV)
report on behalf of the SVOM team:
During the commissioning phase of SVOM mission, we observed the field of EP240918a (Zhang et al., GCN 37541；GCN 37554) with SVOM/VT telescope in ToO mode. The observation started at 2024-09-19T14:11:29 UT, about 26.9 hours after the burst, and the total exposure time was 3820 seconds. VT made the observations with two channels simultaneously, VT_B (400nm-650nm) and VT_R (650nm-1000nm).
No any new optical source was detected within the error circle of EP/FXT (Zhang et al., GCN 37541) down to the 3 sigma upper limit of VT_B=24.0 mag and VT_R=23.6 mag in AB magnitude. The non-detection is consistent with the reports (Wang et al., GCN 37544, Kong et al., GCN 37545, Sun et al., GCN 37546, Fu et al., GCN 37547, Lipunov et al., GCN 37548, Zhang et al., GCN 37568).
Second follow-up was performed started at 2024-09-20T11:14:05 UT, about 48 hours after the trigger. No any new optical source was detected with the similar upper limit.
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 is an optical telescope on board SVOM with an aperture of 44 cm, designed to automatically follow the Eclairs triggers. It has two channels: VT_B and VT_R, covering wavelengths from 400 nm to 650 nm and 650 nm to 1000 nm, respectively. The two bands are observed simultaneously. 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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