TITLE:   GCN CIRCULAR
NUMBER:  37432
SUBJECT: EP240908a: EP detection of a fast X-ray transient
DATE:    24/09/09 09:04:51 GMT
FROM:    EP Team at NAOC/CAS <ep_ta@bao.ac.cn>

X. Mao (NAOC, CAS), J. Q. Peng (IHEP, CAS), S. K. Yang, H. He (WHU), J. W. Hu, W. M. Yuan, C. C. Jin, Y. Liu, Z. X. Ling, C. Zhang, H. Q. Cheng, W. Chen, C. Z. Cui, D. W. Fan, H. B. Hu, M. H. Huang, D. Y. Li, H. Y. Liu, M. J. Liu, Z. Z. Lv, T. Y. Lian, 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), 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 report on the detection of a fast X-ray transient designated EP240908a by the Einstein Probe (EP) mission. The transient event triggered the Wide-field X-ray Telescope (WXT) on-board processing unit and also an automated observation by the on-board Follow-up X-ray Telescope (FXT). The amount of information available around the trigger time is still limited due to delayed data transmission. The exact time of the trigger by WXT is unknown, due to the unsuccessful receipt of the alert message and the delayed data telemetry, but is estimated to occur between 2024-09-08 15:55 and 18:49 UTC. 

Based on the FXT data currently transmitted, the FXT observation began at 2024-09-08T18:49:05 UTC with an exposure time of about 3 ks. An uncataloged X-ray source was detected at R.A. = 14.0031, DEC = 8.0735 (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic). No significant variation in the light curve was observed during the FXT observation. The spectrum can be fitted by an absorbed power-law with NH = 1.5(-1.5/+1.7) x 10^21 cm^-2 and a photon index of 2.1(-0.6/+0.7). The derived unabsorbed 0.5 - 10.0 keV flux is 7.4(-2.0/+1.3) x 10^-13 erg/s/cm^2. A ToO observation with FXT has been planned. No previously known bright X-ray sources have been found within the error circle of the source position. Further information on this source will be updated when more data is available, particularly the WXT data from which the source was first detected. 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).