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ATEL #16509							    ATEL #16509

Title:	Detection of a bright X-ray flare EPW20240305aa by Einstein
		Probe
Author:	Y. Liu (NAOC, CAS), M. J. Liu (NAOC, CAS), H. Q. Cheng (NAOC,
		CAS), W. D. Zhang (NAOC, CAS), C. Zhang (NAOC, CAS), Z. X. Ling (NAOC,
		CAS), C. C. Jin (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), H. Y. Liu (NAOC, CAS), Z. Z. Lv (NAOC, CAS), X. Mao (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), W. J. Zhang (NAOC, CAS), Z. Zhang (NAOC, CAS),
		D. H. Zhao (NAOC, CAS), E. Kuulkers (ESA), P. O'Brien (Univ. of
		Leicester), S. N. Zhang (IHEP, CAS) and W. Yuan (NAOC, CAS), on behalf
		of the Einstein Probe team
Queries:	ep_ta@bao.ac.cn
Posted:	7 Mar 2024; 11:31 UT
Subjects:X-ray, Transient

We report on the detection of a bright X-ray flare EPW20240305aa at 2024-03-05T14:15:31
(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. = 122.903 deg, DEC = -54.657 deg (J2000) with an uncertainty of
3 arcmin (radius, 90% C.L. statistical and systematic). The light curve
of the source shows a double-peak profile. The source first exhibits a
rise to a peak 0.5 - 4.0 keV flux of approximately 5 x 10^-9 erg/s/cm^2
in about 100 seconds, then quickly diminishes to the background level of
~1 x 10^-11 erg/s/cm^2 within about 50 seconds. A second, weaker flare
follows after about 200 seconds, showing a fast rise to a peak flux of
~2.5 x 10^-9 erg/s/cm^2 within about 50 seconds and a relatively slower
decay to the background level within about 200 seconds. The averaged spectrum
within the interval of these two flares can be fitted by an absorbed power-law
with NH = 3.4(-1.7/+1.8) x 10^21 cm^-2 and a photon index of 1.6(-0.5/+0.5).
The derived average unabsorbed 0.5 - 4.0 keV flux is 8.3(-1.4/+2.0) 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 around this position.
Within the 3 arcmin region around the source position, only one X-ray source
was found in the first six months data of the SRG/eROSITA all-sky survey,
but its X-ray flux of 2.24e-14 erg/s/cm^2 is much lower than that of EPW20240305aa.
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 partially activated WXT instrument
during its commissioning phase. 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.

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