TITLE: GCN CIRCULAR NUMBER: 40696 SUBJECT: EP250610a: continuous follow-up observations with EP-FXT DATE: 25/06/12 08:52:14 GMT FROM: EP Team at NAOC/CAS ep_ta@bao.ac.cn
H. Q. Cheng (NAO, CAS), Y. J. Zhang (THU) , T. Y. Lian, C. C. Jin (NAO, CAS) report on behalf of the Einstein Probe team:
Following the detection of the fast X-ray transient EP250610a (Zhang et al., GCN 40660, Lian et al. GCN 40669) and its multi-wavelength follow-up observations (Schneider et al., GCN 40661, Evans et al., GCN 40662, Brivio et al., GCN 40665, Yao et al., GCN 40666, Lipunov et al., GCN 40673, Eyles-Ferris et al., GCN 40686, Brivio et al., GCN 40688, Siegel et al., GCN 40690), we performed two target-of-opportunity (ToO) observations with the Follow-up X-ray Telescope (FXT) on board the Einstein Probe mission. The X-ray counterpart detected in the autonomous follow-up observation (Lian et al. GCN 40669) as well as the Swift follow-up observation (Evans et al., GCN 40662) was detected in both epochs.
The first ToO observation began at 2025-06-10 15:36:01 (UTC), about 11 hours after the EP-WXT detection. The exposure time is 2975 seconds. Preliminary analysis shows that the 0.5-10 keV FXT spectrum can be fitted with an absorbed power-law model with NH fixed at the Galactic value of 9.5e19 cm^-2 and a photon index of 1.97(-0.77, +0.86). The unabsorbed 0.5-10 keV flux is 1.13 (-0.53, +1.24) e-13 erg/s/cm^2 (90% C.L.), about one order of magnitude lower than that measured in the autonomous follow-up observation (Lian et al. GCN 40669).
The second ToO observation began at 2025-06-11 15:35:15 (UTC), about 35 hours after the EP-WXT detection. The exposure time is 5950 seconds. Preliminary analysis shows that the 0.5-10 keV spectrum of this epoch can be fitted with an absorbed power-law model with NH fixed at the Galactic value of 9.5e19 cm^-2 and a photon index of 1.39(-0.94, +0.83), yielding an unabsorbed 0.5-10 keV flux of 6.91(-4.42, +14.50)e-14 erg/s/cm^2 (90% C.L.). The uncertainties are given at 90 percent confidence level for all the above parameters.
EP-FXT will continue monitoring the source in the forthcoming days. Multi-band follow-up observations are encouraged. The contact TAs of EP250610a are Tianying Lian and Huaqing Cheng, please contact them via the email tylian@nao.cas.cn and hqcheng@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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