TITLE: GCN CIRCULAR NUMBER: 37554 SUBJECT: EP240918a: EP observation update DATE: 24/09/19 08:42:10 GMT FROM: EP Team at NAOC/CAS ep_ta@bao.ac.cn
Z. J. Zhang (HKU), Y. F. Liang (PMO, CAS), H. Y. Liu, W. Chen, T. Zhao , Z. X. Ling, J. W. Hu, W. M. Yuan, C. C. Jin, Y. Liu, C. Zhang, H. Q. Cheng, C. Z. Cui, D. W. Fan, H. B. Hu, M. H. Huang, D. Y. Li, M. J. Liu, Z. Z. Lv, T. Y. Lian, X. Mao, H. W. Pan, X. Pan, H. Sun, W. X. Wang, Y. L. Wang, X. P. Xu, Y. F. Xu, H. N. Yang, M. Zhang, W. D. Zhang, W. J. Zhang, Z. Zhang, D. H. Zhao (NAOC, CAS), X. F. Zhao, Y. Chen, S. M. Jia, W. W. Cui, D. W. Han, C. K. Li, L. M. Song, J. Zhang, S. N. Zhang (IHEP, CAS), E. Kuulkers, A. Santovincenzo (ESA), P. O'Brien (Univ. of Leicester), K. Nandra, A. Rau (MPE), B. Cordier (CEA) on behalf of the Einstein Probe team
We have reported the detection of the fast X-ray transient EP240918a by the Einstein Probe (EP) mission (Zhang et al. GCN 37541). Based on the newly received EP telemetry data of this source, the transient event started at 2024-09-18T11:21:52 (UTC). The lightcurve of the transient obtained by the WXT lasts around 170 seconds and has a peak flux of around 3.2 x 10^(-9) erg/s/cm^2 in the 0.5-4 keV band. The average 0.5-4 keV spectrum can be fitted with an absorbed power law with a photon index of 1.7 (-0.6, +0.7) (with the column density fixed at the Galactic one of 9.9 x 10^20 cm^-2). The derived average unabsorbed 0.5-4 keV flux is 7.2 (-1.9, +2.8) x 10^(-10) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.
The EP-FXT autonomous follow-up observation started at 2024-09-18T11:25:55 (UTC). The observation clearly shows an uncatalogued X-ray source located at R.A. = 289.3933 deg, DEC = 46.1278 deg (J2000) with an uncertainty of 10 arcsec in radius (90% C.L. statistical and systematic). The EP-FXT light curve shows a fast decline and the flux decreased to around 1.0 x 10^(-13) erg/s/cm^2 within about 1000 s. Further follow-up observations are encouraged to identify the nature of this X-ray transient.
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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