We report on the detection of a bright X-ray flare EPW20240219aa at 2024-02-19 06:21:42 (UTC) by one of the Wide-field X-ray Telescope (WXT) modules on board the Einstein Probe (EP) mission. The position of the source is R.A. = 80.016 deg, DEC = 25.541 deg (J2000) with an uncertainty of 3 arcmin (radius, 90% C.L. statistical and systematic).

The source exhibits a fast rise to a peak 0.5 - 4.0 keV flux of approximately 5 x 10^-9 erg/s/cm^2 in about 10 seconds, followed by a relatively slow decay to the background level of 4 x 10^-11 erg/s/cm^2 within about 200 seconds. The averaged spectrum can be fitted by an absorbed power-law with NH = 8.6(-0.4/+0.5) x 10^21 cm^-2 and a photon index of 2.0(-0.8/+0.9). The derived average unabsorbed
0.5 - 4.0 keV flux is 6.9(-2.1/+5.6) x 10^-10 erg/s/cm^2. However, it should be noted the derived parameters of the source may be subject to larger uncertainties than those quoted here since in-orbit calibration of the instrument is still in progress.

No previously known X-ray sources are found around this position. We performed a Swift target of opportunity observation which began at 2024-02-20 21:31:41(UTC) with an exposure time of 980 seconds in Photon Counting mode. No sources were detected within the position error circle of EPW20240219aa. More follow-up observations are encouraged to identify the nature of this X-ray flare.

We thank the Neil Gehrels Swift Observatory team for making the ToO observation possible.

The above observation was made with the partially activated WXT instrument during its commissioning phase.

The preliminary results have been posted at Astronomer's Telegram as ATel 16463.  Click here for more EP/LEIA Astronomer's Telegrams.