We report on the detection of a bright X-ray flare EP240305a (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.
The preliminary results have been posted at Astronomer's Telegram as ATel 16509. Click here for more EP/LEIA Astronomer's Telegrams.
Related GCN/ATel report:
Swift follow-up observation of the EP X-ray transient EP240305a (EPW20240305aa)