GCN 44507: GRB 260509A: SVOM/VT optical candidate with VHF data

2026-05-10T01:47:02.038Z | rev 2 | event: GRB 260509A
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. 176.89974, 0.28822 degrees, corresponding to:  
      R.A. (J2000) = 11:47:35.94   
      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.