GCN Circular 38778

Y. F. Liang (PMO, CAS), A. Li (BNU), H. L. Peng (NNU), W. F. Wen(SATU), W. D. Zhang (NAOC, CAS) on behalf of the Einstein Probe team:

We report on the detection of two X-ray transients designated EP241231b and EP250101a, respectively, by the Einstein Probe (EP) mission. 

EP241231b was detected by EP-WXT at 2024-12-31T19:27:29 (UTC). The WXT position is R.A.= 100.064 deg, DEC= 16.171 deg (J2000) with an uncertainty of about 2.6 arcmin (radius, 90% C.L. statistical and systematic), corresponding to Galactic l, b = 197.255 deg, 4.893 deg. The absorbed average flux in 0.5-4 keV is 1.5(+1.1/-0.5) x 10^(-10) erg/s/cm^2 with column density fixed at the Galactic value 4.1 x 10^21 cm^-2 and a photon index of 2.1(+1.1/-0.9). 
 
EP250101a was detected by EP-WXT at 2025-01-01T05:52:21 (UTC). The WXT position is R.A.=85.575 deg, DEC=0.352 deg (J2000) with an uncertainty of about 3.2 arcmin (radius, 90% C.L. statistical and systematic), corresponding to Galactic l, b = 204.535 deg, -15.167 deg. A flare with a duration of around 2,500 seconds is visible in the WXT lightcurve. The peak flux of the X-ray is ~ 1.7 x 10^(-10) erg/s/cm^2 in the 0.5-4 keV band. The average 0.5-4 keV spectrum can be fitted with an absorbed power law with a photon index of 1.5(+1.6/-1.1) (with column density fixed at the Galactic value 3.3 x 10^21 cm^-2). The derived average unabsorbed 0.5-4 keV flux is 4.4(+2.3/-1.7) x 10^(-11) erg/s/cm^2. The quoted uncertainties are at the 90% confidence level for the above parameters.

No previously known bright X-ray sources are found within the error circle around the source positions of either EP241231b or EP250101a. Following the WXT detections, we performed two follow-up observations, one for each transient, with the Follow-up X-ray Telescope (FXT) on board EP. Further follow-up observations are encouraged to identify the nature of these X-ray transients.  

Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with onboard X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).