GCN 43942: GRB 260307B: SVOM/VT optical candidate

J. T. Palmerio (CEA/Irfu), H. L. Li, L. P. Xin, Y. N. Ma, Y. L. Qiu, C. Wu (NAOC), A. Saccardi (CEA/Irfu), F. Magnani, A. Foisseau (CPPM)
on behalf of the SVOM/VT team.

After the trigger by SVOM/ECLAIRs at 2026-03-07T13:45:26 UTC (T0), SVOM performed an automatic slew on the burst location (Magnani et al., GCN 43940). SVOM/VT began observing the field at 2026-03-07T13:51:31, 364.31 seconds after T0, 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 downloaded via VHF network, at least one credible candidate is identified, the details of which are presented below.

VT_ID 245: 
 This candidate was flagged as an uncatalogued fading source.
 The position of this candidate is R.A., Dec. 155.5129, -21.0067 degrees, corresponding to:
  R.A. (J2000) = 10h22m03.1s
  Dec. (J2000) = -21d00m24.1s
 with an uncertainty of 0.5 arcsec.
 This location is within the R90 uncertainty region of the SVOM/MXT onboard localization.
 The source was detected only in VT_R and faded beyond detection in the third sequence.
 The candidate's magnitudes are:

 | date-obs (UTC)      | mid-time | exposure | band | mag(AB)    |
 | ------------------- | -------- | -------- | ---- | ---------- |
 | 2026-03-07T13:51:31 |  6.1 min | 6*50 sec | VT_R | 21.3 ± 0.1 |
 | 2026-03-07T13:56:31 | 11.1 min | 6*50 sec | VT_R | 21.8 ± 0.1 |

 Magnitudes were not corrected for Galactic extinction. 

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.