TITLE: GCN CIRCULAR NUMBER: 44507 SUBJECT: GRB 260509A: SVOM/VT optical candidate with VHF data DATE: 26/05/10 01:47:02 GMT FROM: SVOM_group <svomgroup@bao.ac.cn> C. Wu, H. L. Li, Y. L. Qiu, L. P. Xin (NAOC), J. T. Palmerio (CEA/Irfu), D. Götz (CEA/Irfu), A. Saccardi (CEA/Irfu), L. Zhang (IHEP), Y. H. Cheng (YNU) on behalf of the SVOM/VT team. After the trigger by SVOM/ECLAIRs (SVOM burst-id sb26050905) at 2026-05-09T21:51:09 UTC (T0), SVOM/VT began observing the field after the automatic slew on the burst (Götz et al., GCN 44504) in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously. From a preliminary analysis of the 1-bit subimage and source list generated by on-board data processing and downloaded via VHF network, one credible candidate is identified, the details of which are presented below. VT_ID 722: The position of this candidate is R.A., Dec. 176.8997, 0.2882 degrees, corresponding to: R.A. (J2000) = 11:47:35.9 Dec. (J2000) = 00:17:17.6 with an uncertainty of 0.50 arcsec. This candidate was detected in both VT_R and VT_B, and was flagged as an uncatalogued source whose brightness faded during the VT observations. The candidate's magnitudes are: | date-obs (UTC) | mid-time | exposure | VT_B mag(AB) | VT_R mag(AB) | | -------------------- | ----------- | --------- | ------------- | ------------- | | 2026-05-09T23:18:31 | 89.87 min | 6x50 sec | 20.38 ± 0.03 | 19.80 ± 0.04 | | 2026-05-09T23:58:59 | 129.08 min | 3x50 sec | 20.67 ± 0.05 | 20.18 ± 0.06 | Magnitudes were not corrected for Galactic extinction. The mean color of VT_B - VT_R = 0.54 is in line with the expectations of an event at redshift z < 4. The position of this candidate is consisent with the reports (O'Neill et al., GCN 44505; Saccardi et al., GCN 44506) . The Space-based multi-band astronomical Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA), French Space Agency (CNES), and the Chinese Academy of Sciences (CAS), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. SVOM/VT was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC), CAS. View this GCN Circular online at https://gcn.nasa.gov/circulars/44507. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...