TITLE: GCN CIRCULAR NUMBER: 40448 SUBJECT: EP250512a/GRB 250512B refined analysis of the EP-WXT and EP-FXT observations DATE: 25/05/13 07:02:52 GMT FROM: EP Team at NAOC/CAS ep_ta@bao.ac.cn

H.N. Yang, D. H. Zhao (NAO, CAS), Y. L. Hua (PMO, CAS), Y. J. Zhang (THU), M. J. Liu, J. W. Hu, H. W. Pan, Y. Liu (NAO, CAS), on behalf of the Einstein Probe (EP) team:

The X-ray transient EP250512a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (D. H. Zhao et al. GCN 40437). It was also detected by SVOM/ECLAIRs (Pierre Maggi et al. GCN 40439), and followed up by several telescopes ( H. L. Li et al. GCN 40443; L. P. Xin et al. GCN 40444; V.Lipunov et al. GCN 40446; Malte Busmann et al. GCN 40447). Refined analysis of the WXT data shows that the event started at T0 = 2025-05-12T11:20:03 (UTC) and lasted for about 360 s (T90). The peak flux is about 4.2 x 10^(-9) erg/s/cm^2 (T0+135 s). The averaged 0.5-4 keV spectrum of T90 can be fitted by an absorbed power law model with a Galactic hydrogen column density of 5.7 x 10^20 cm^-2 and a photon index of 1.31 (-0.17/+0.18). The derived average unabsorbed 0.5-4 keV flux is 1.13 (-0.13/+0.14) x 10^(-9) erg/s/cm^2. The uncertainties are quoted at the 90% confidence level for the above parameters.

The Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously about 4 min after T0, starting at 2025-05-12T11:23:44 (UTC). Within the WXT error circle, on-ground analysis of the FXT data found an uncatalogued source at R.A. = 215.2043, DEC = -10.1009 (J2000) with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic). During the observation a decline in the source flux lasting for a few hundred seconds is observed from the beginning. The averaged 0.5-10 keV FXT spectrum (from T0+291 s to T0+7094 s, with a total exposure time of 4,095 s) can be fitted by an absorbed power law model with a Galactic hydrogen column density of 5.7 x 10^20 cm^-2 and a photon index of 1.45 (+/-0.05). The derived average unabsorbed 0.5-10 keV flux is 3.78 (-0.18/+0.19) x 10^(-11) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.

EP-FXT will continue monitoring this source. More follow-up observations are encouraged. The contact TAs of EP250512a are D. H. Zhao and H. N. Yang. Please contact them via email zhaodh@bao.ac.cn, hnyang@nao.cas.cn if needed.

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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