TITLE: GCN CIRCULAR NUMBER: 40170 SUBJECT: GRB 250419A: SVOM/VT optical bright counterpart confirmation DATE: 25/04/19 05:05:51 GMT FROM: Liping Xin at NAOC, SVOM xlp@nao.cas.cn

L. P. Xin, Y. L. Qiu, C. Wu, H. L. Li., Z. H. Yao, Y. N. Ma, X. H. Han, H. B. Cai,J.Y. Wei (NAOC), J. T. Palmerio (CEA), B.-T. Wang (YNAO, CAS), Y.H. Cheng (SWIFAR,YNU), W. J. Xie(NAOC), report on behalf of SVOM/VT team:

After the trigger by SVOM/ECLAIRs at 2025-04-19T02:29:32 UTC (Tb), SVOM performed an automatic slew on the burst. SVOM/VT began observing the field at 45.26 min after the SVOM trigger, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.

An uncatalogued and bright source, compared to DESI catalog, is found by automatic ground pipeline based on VHF data within the error box of SVOM/ECLAIRs (Wang et al. GCN 40168) at ra=202.40533, dec=7.04106 (J2000), corresponding to:

RA (J2000) = 13:29:37.28 Dec (J2000) = +07:02:27.8 with an uncertainty of 0.5 arcsec.

The source is detected in both VT_R and VT_B in the third VT observing sequence, the magnitudes are given below:

MidTime min | exposure time (s) | mag(AB) | mag err | band 47.767 | 6x50 | 17.790| 0.02 | VT_R 47.767 | 6x50 | 18.220| 0.01 | VT_B

The position and the brightness are consistent with the report (López et al., GCN 40169).

Given the VT color of the counterpart, it might be a low redshift gamma-ray burst.

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.

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