TITLE: GCN CIRCULAR NUMBER: 40474 SUBJECT: GRB 2500515A/GOTO25cqo: SVOM/VT optical blue color DATE: 25/05/17 01:24:43 GMT FROM: Huali Li at at NAOC, SVOM lhl@nao.cas.cn
L. P. Xin, Y. L. Qiu, H. L. Li, C. Wu, Z. H. Yao, Y. N. Ma, X. H. Han, Y. Xu, J. Wang, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, L. Lan, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA), Y. Wang (PMO, CAS), R. Z. Li (YNAO, CAS), J. X. Cao, D. F. Kong (GXU), J. R. Mao (YNAO, CAS), X. W. Liu (YNU) report on behalf of the SVOM/VT team:
SVOM/VT conducted ToO follow-up observations of the optical transient (B. P. Gompertz et al., GCN 40464) which was proposed to be the optical candidate of the GRB 250515A (Fermi GBM team, GCN 40462). The observation started from 2025-05-16T12:16:52 UTC (15.50 hours after the Fermi GBM trigger) to 2025-05-16T19:20:30 UTC in VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.
The optical candidate (B. P. Gompertz et al., GCN 40464) was clearly detected in VT both channels, which is not found in the legacy survey.
The brightness in AB magnitude was estimated to be: Mid time (hour) | Band | Exposure Time (second) | Magnitude | Magnitude error 22.27 | VT_B | 32x70 | 20.25 | 0.03 22.27 | VT_R | 32x70 | 20.56 | 0.05
Our photometry was not corrected for Galactic extinction.
The source shows no significant fading or brightening in both channels during our observations. However, given the blue color and the possible association to the likely short GRB (Fermi GBM team, GCN 40462), more multiband follow-up observations are encouraged.
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 was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC),CAS.
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