------------------------------------------------------------------------------

The Astronomer's Telegram                   http://www.astronomerstelegram.org

==============================================================================
ATEL #16564							    ATEL #16564

Title:	A fast X-ray transient EP240331a detected by Einstein Probe
Author:	X. Pan, M. J. Liu, Y. Liu, C. Zhang, Z. X. Ling,  C. C. Jin,
		H. Q. Cheng, W. Chen, C. Z. Cui, D. W. Fan, H. B. Hu, J. W. Hu, M.
		H. Huang, D. Y. Li, T. Y. Lian, H. Y. Liu, Z. Z. Lv, X. Mao, H. W.
		Pan, H. Sun, W. X. Wang, Y. L. Wang, Q. Y. Wu, X. P. Xu, Y. F. Xu,
		H. N. Yang, W. Yuan, M. Zhang,  W. D. Zhang, W. J. Zhang, Z. Zhang,
		D. H. Zhao (NAOC, CAS), Y. Chen, W. W. Cui, H. Feng, J. Guan, D. W.
		Han, S. M. Jia, C. K. Li, W. Li, C. Z. Liu, F. J. Lu, L. M. Song,
		J. Wang, J. J. Xu, J. Zhang, S. N. Zhang, H. S. Zhao, X. F. Zhao,
		(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
Queries:	ep_ta@bao.ac.cn
Posted:	1 Apr 2024; 16:14 UT
Subjects:X-ray, Transient

We report on the detection of a fast X-ray transient EP240331a at 2024-03-31T22:07:17
(UTC) by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe
(EP) mission during a calibration observation. The source position is R.A.
= 169.414 deg, DEC = -20.042 deg (J2000) with an uncertainty of 20 arcmin
(radius, 90% C.L. statistical and systematic; the large positioning uncertainty
is due to the so far incomplete calibration of the CMOS detector which
detected the source). The light curve shows a symmetric profile with a
duration of around 100 s. The averaged spectrum of the flare can be fitted
by an absorbed powerlaw with a photon index of 0.74(-0.30/+0.31) (with
absorption NH fixed at the Galactic value 3.44 x 10^20 cm^-2). The derived
average unabsorbed 0.5-4 keV flux is 1.8(-0.3/+0.3) x 10^-9 erg/s/cm^2.
No previously known bright X-ray sources are found around this position.
Further multi-wavelength follow-up observations are 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.

------------------------------------------------------------------------------