TITLE: GCN CIRCULAR NUMBER: 37585 SUBJECT: EP240919a: EP-FXT follow-up observation update DATE: 24/09/21 12:26:54 GMT FROM: EP Team at NAOC/CAS ep_ta@bao.ac.cn

Y. F. Liang (PMO, CAS), M. J. Liu, H. Q. Cheng, X. Pan, X. P. Xu, H. N. Yang(NAOC, CAS), Z. J. Zhang (HKU), C. Y. Dai (NJU), C. X. Zhang (HUST), C. C. Jin, H. W. Pan, Z. X. Ling, W. M. Yuan, Y. Liu, C. Zhang, W. Chen, C. Z. Cui, D. W. Fan, H. B. Hu, J. W. Hu, M. H. Huang, D. Y. Li, H. Y. Liu, Z. Z. Lv, T. Y. Lian, X. Mao, H. Sun, W. X. Wang, Y. L. Wang, Y. F. Xu, M. Zhang, W. D. Zhang, W. J. Zhang, Z. Zhang, D. H. Zhao (NAOC, CAS), Y. Chen, S. M. Jia, W. W. Cui, D. W. Han, C. K. Li, L. M. Song, X. F. Zhao, 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 EP240919a by the Einstein Probe (EP) mission (Liang et al. GCN 37561). An automonous follow-up observation with the EP-FXT was triggered by the onboard processing and triggering system of EP, starting at 2024-09-19T14:57:50 (UTC), 4 minutes after the onboard triggering. An uncatalogued X-ray source is clearly detected, at R.A. = 334.2790 deg, DEC = -9.7361 deg (J2000) with an uncertainty of 10 arcsec in radius (90% C.L. statistical and systematic). The source position is consistent with that of the WXT transient within the uncertainties. The EP-FXT light curve shows a fast decline and the flux decreased to around 1.0 x 10^(-12) erg/s/cm^2 within about 400 s. The average 0.5-10 keV spectrum can be fitted with an absorbed power-law with a photon index of 2.0(+0.4/-0.4) (with the column density fixed at the Galactic value of 5.4 x 10^20 cm^-2). The absorbed average flux in 0.5-10 keV is 5.8(+2.2/-1.4) x 10^(-12) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.

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 onboard X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).

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