TITLE: GCN CIRCULAR NUMBER: 40299 SUBJECT: EP250430a/GRB 250430A: Einstein Probe WXT detection of the X-ray prompt emission DATE: 25/05/01 08:11:44 GMT FROM: EP Team at NAOC/CAS ep_ta@bao.ac.cn

C. Y. Wang (THU), H. L. Peng (NJNU), Y. Q. Zhao (USTC, PRIC), W. Yuan (NAO, CAS), behalf of the Einstein Probe (EP) team:

We report on the detection of a fast X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP250430a. The WXT detection was not triggered onboard due to the closeness of the source position to the bright X-ray source Sco X-1, but was made in the onground analysis of the telemetry data later. No automated follow-up X-ray observation was performed. The WXT position of the source is R.A. = 233.399 deg, DEC = -18.130 deg (J2000) with an uncertainty of 2.9 arcmin in radius (90% C.L. statistical and systematic), which is offset by 1.0 arcmin from the Swift/XRT position of the likely long GRB 250430A (Lipunov et al., GCN 40291; Parsotan et al., GCN 40292; Kawabata et al., GCN 40294; Odeh et al., GCN 40295). The X-ray rise time precedes the GRB trigger time by approximately 23 seconds, and the X-ray duration is longer than that of the GRB by about 30 seconds. The X-ray emission peaked at 2025-04-30T17:31:05.5 (UTC). The consistency of EP250430a and GRB 250430A in position and time suggests EP250430a to be the X-ray counterpart of GRB 250430A. Note that the source parameters given above are only approximate and detailed analysis is onging.

The averaged WXT spectrum in 0.5-4 keV can be fitted by an absorbed power law model with a photon index of 2.37 (+0.61, -0.54) and a column density of 3.80 (+1.50, -1.31) x 10^21 cm^-2. The average unabsorbed 0.5-4 keV flux is about 1.63 (+0.69, -0.36) x 10^-10 erg/cm^2/s. These parameters derived are at the 1-sigma confidence level.

Further observations with the Follow-up X-ray Telescope (FXT) on board EP may be considered at a later stage. Contact transient advocate (TA) for EP250430a is C. Y. Wang (wang-cy22@mails.tsinghua.edu.cn) and please contact the TA for information regarding the EP observation of this source.

Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).

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