TITLE:   GCN CIRCULAR
NUMBER:  35931
SUBJECT: Einstein Probe detected of a fast X-ray transient EP240315a
DATE:    24/03/16 15:49:37 GMT
FROM:    EP Team at NAOC/CAS <ep_ta@bao.ac.cn>

W. J. Zhang (NAOC, CAS), X. Mao (NAOC, CAS), W. D. Zhang (NAOC, CAS), H. Y. Liu (NAOC, CAS), Y. Liu (NAOC, CAS), C. Zhang (NAOC, CAS),

Z. X. Ling (NAOC, CAS), C. C. Jin (NAOC, CAS), H. Q. Cheng (NAOC, CAS), W. Chen (NAOC, CAS), C. Z. Cui (NAOC, CAS), D. W. Fan (NAOC, CAS),

H. B. Hu (NAOC, CAS), J. W. Hu (NAOC, CAS), M. H. Huang (NAOC, CAS), D. Y. Li (NAOC, CAS), T. Y. Lian (NAOC, CAS), M. J. Liu (NAOC, CAS),

Z. Z. Lv (NAOC, CAS), H. W. Pan (NAOC, CAS), X. Pan (NAOC, CAS), H. Sun (NAOC, CAS), W. X. Wang (NAOC, CAS),Y. L. Wang (NAOC, CAS),

Q. Y. Wu (NAOC, CAS), X. P. Xu (NAOC, CAS), Y. F. Xu (NAOC, CAS), H. N. Yang (NAOC, CAS), M. Zhang (NAOC, CAS), S. N. Zhang (IHEP, CAS),

Z. Zhang (NAOC, CAS), D. H. Zhao (NAOC, CAS), E. Kuulkers (ESA), P. O'Brien (Univ. of Leicester) and W. Yuan (NAOC, CAS), on behalf of the Einstein Probe team

 

 


  We report on the detection of a fast X-ray transient EP240315a at 2024-03-15T20:10:44 (UTC) by the Wide-field X-ray Telescope (WXT) on board

the Einstein Probe (EP) mission during a calibration observation. The position of the source is R.A. = 141.644 deg, DEC = -9.547 deg (J2000)

with an uncertainty of 3 arcmin (radius, 90% C.L. statistical and systematic). The light curve of the source shows a multi-peak profile. 

The transient event lasts for ~1600 seconds and has a peak flux of ~3e-9 erg/s/cm^2 in the 0.5-4 keV band. The averaged spectrum can be fitted

by an absorbed power-law with NH = 1.5(-0.9/+1.0) x 10^21 cm^-2 and a photon index of 1.7(-0.4/+0.4). The derived average unabsorbed 0.5 - 4.0 keV

flux is 5.3(-0.7/+1.0) x 10^-10 erg/s/cm^2. However, we note that the derived source parameters may be subject to larger uncertainties than those

quoted here since in-orbit calibration of the instrument is still in progress.

 


  No previously known bright X-ray sources have been found within the 3 arcmin region around the source position, with only one optical counterpart

of white dwarf candidate located at the distance of ~380 pc. Based on the shape and timescale of the observed flare light curve, we tend to consider

that the source is not a stellar flare, although this cannot be ruled out. A Swift target of opportunity observation has been proposed, and further

follow-up observations are strongly encouraged to identify the nature of this X-ray flare.





  The above observation was made with the WXT instrument during the commissioning phase of EP. 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). EP is a mission of the Chinese

Academy of Sciences in collaboration with ESA, MPE and CNES.