TITLE:   GCN CIRCULAR
NUMBER:  38281
SUBJECT: EP241119a: EP detection of a fast X-ray transient
DATE:    24/11/20 07:19:43 GMT
FROM:    EP Team at NAOC/CAS <ep_ta@bao.ac.cn>

W. J. Zhang (NAO, CAS), Q. C. Shui (IHEP, CAS), H. Z. Wu (HUST), H. He, S. K. Yang (WHU), Y. L. Hua (PMO, CAS), W. Yuan (NAO, CAS) on behalf of the Einstein Probe team

We report on the detection of a fast X-ray transient, designated EP241119a, by the Wide-field X-ray Telescope (WXT) and confirmed by the Follow-up X-ray Telescope (FXT) on board the Einstein Probe (EP) mission. The transient was first detected with WXT at 2024-11-19 17:53:20 (UTC) and lasted for around 200 seconds. The WXT position of EP241119a is R.A.= 84.116 deg, DEC = 3.832 deg (J2000) with an uncertainty of 2.3 arcmin (radius, 90% C.L. statistical and systematic). It has a peak flux of ~4 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 1.27(+0.45/-0.44) (with a column density fixed at the Galactic value of 2.1 x 10^21 cm^-2). The derived average unabsorbed 0.5-4 keV flux is 2.43(+0.67/-0.52) x 10^(-10) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters. 

At the detection of EP241119a EP was performing a calibration observation for FXT, during which the FXT automated follow-up mode was disabled. About 9 hours later a follow-up observation was carried out with FXT via ground-satellite command uplink. An uncataloged X-ray source was detected at R.A. = 84.1062 deg, DEC = 3.8404 deg (J2000) with an uncertainty of about 10 arcsecs (radius, 90% C.L. statistical and systematic), consistent with the position of the WXT transient within the uncertainties. The source flux is estimated to be around 2 x 10^(-13) erg/s/cm^2.

No previously known bright X-ray sources are found within the error circle around the source position. 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).