EP240618a: EP-WXT detection of a fast X-ray transient and EP-FXT quick follow-up
Abstract
We report on the detection of a fast X-ray transient, designated EP240618a, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The transient triggered the WXT on-board processing unit at 2024-06-18T05:43:43 (UTC). The position of the source is R.A. = 281.627 deg, DEC = 23.820 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic). The transient event lasts for approximately 100 seconds and has a peak unabsorbed flux of ~1.5 x 10^-8 erg/s/cm^2 in the 0.5-4 keV band. The average 0.5-4 keV spectrum can be fitted by an absorbed power law with a photon index of 1.2(+/-0.4) (with the column density fixed at the Galactic value of 1.98 x 10^21 cm^-2). The derived average unabsorbed 0.5-4 keV flux is 2.9(+0.7/-0.6) x 10^-9 erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters. No previously known X-ray sources at a similar flux level are found within the 3 arcmin region around the source position. Following the WXT detection, we performed a follow-up observation with the Follow-up X-ray Telescope (FXT) on board the EP, which started at 2024-06-18T06:55:00 (UTC), about 1 hour after the WXT detection. An X-ray source was clearly detected at R.A. = 281.648 deg, DEC = 23.833 deg (R.A. = 18:46:35.4, Dec = 23:49:57.0; J2000), with an uncertainty of 30 arcsec (radius, 90% C.L. statistical and systematic), which is 1.4 arcmin away from the WXT position of EP240618a. The spectrum can be fitted with an absorbed power-law model with NH fixed at the Galactic value of 1.98 x 10^21 cm^-2 and a photon index of 2.0(+/-0.1). The derived unabsorbed flux in 0.5-10 keV is 3.0(+/-0.3) x 10^-12 erg/s/cm2. During the observation, the flux of the source was decaying with time. Considering this source being within the error circle of the WXT transient, we suggest it being the late-time X-ray emission from EP240618a. Further follow-up observations are encouraged to identify the nature of this X-ray transient. The above observations were made with the WXT and FXT instruments 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.
- Publication:
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GRB Coordinates Network
- Pub Date:
- June 2024
- Bibcode:
- 2024GCN.36690....1S