TITLE:   GCN CIRCULAR
NUMBER:  45575
SUBJECT: GRB 260915A: SVOM/VT optical upper limits from VHF data
DATE:    26/09/15 05:00:39 GMT
FROM:    SVOM_group <svomgroup@bao.ac.cn>

C. Wu, Y. N. Ma, L. P. Xin, Y.L. Qiu, H.L. Li, J.R. Xu(NAOC),  J. T. Palmerio (CEA/Irfu), W. Tan (IHEP) report on behalf of the SVOM/VT team.

After the trigger by SVOM/ECLAIRs at 2026-09-15T01:12:03   UTC (T0), SVOM performed an automatic slew on the burst location ( Liang et al., GCN 45570). SVOM/VT took images of the field 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 the VHF network, no uncatalogued optical source was detected within the SVOM/MXT (Liang et al., GCN 45570) localization region in our three sequence images, compared to the Pan-STARRS1 DR1. The 5-sigma upper limits are:
| start time (min) | Band | Exposure (s) | 5-sigma upper limit (mag) |
| :--- | :--- | :--- | :--- |
| 5.42 | VT_B | 6 * 50 | > 21.5 |
| 5.42 | VT_R | 6 * 50 | > 21.5 |
| 11.25 | VT_B | 6 * 50 | > 21.5 |
| 11.25 | VT_R | 6 * 50 | > 21.5 |
| 81.48 | VT_B | 6 * 50 | > 21.5 |
| 81.48 | VT_R | 6 * 50 | > 21.5 |


The non-detection is consistent with Yin et al. (GCN 45573). Radio and infrared follow-up observations are encouraged.


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.