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GCN Circular 36025

GRB/FXT 240402B: VLT/FORS2 optical afterglow candidate
2024-04-04T14:40:14Z (16 days ago)
Andrew Levan at Radboud University <>
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A.J. Levan (Radboud), D.B. Malesani (DAWN/NBI & Radboud), P.G. Jonker (Radboud), N.R. Tanvir (Leicester), M. E. Ravasio (Radboud), A. Martin-Carrillo (UCD), G. Pugliese (Amsterdam), S. Campana (INAF/Brera), D. Xu (NAOC/CAS) report on behalf of the Stargate collaboration: 

We obtained observations of FXT/GRB 240402B (Xu et al., GCN 36016; Xue et al., GCN 36017) with the FORS2 imager on the VLT UT1 (Antu). Observations began at 06:44 UT on 4 April 2024, approximately 1.9 days after the high-energy detection. We obtained a total of 1400 s of observations in the R band. Within the 10” error box of the X-ray transient located by Einstein Probe follow-up observations (Jia et al., GCN 36022), we identify a new source not present in archival images retrieved via the Legacy Survey. This source lies at 

RA = 16:21:48.247
Dec = +25:45:47.34

We suggest this is the optical afterglow of FXT/GRB240402B. Photometry is complicated by the presence of a nearby star just 1” away (a confirmed star, given a Gaia measured proper motion), but the magnitude of the transient at the time of the observations was approximately r ~ 22 calibrated to Pan-STARRS. The offset from the nearby galaxy CGCG 138-001 (z = 0.047) is approximately 200 kpc in projection, at the higher end of those expected for ejected compact object mergers.

We thank the staff at the VLT for the rapid execution of these observations, in particular Julia Seidel.

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