GCN Circular 37555

Y. F. Liang (PMO, CAS), Z. J. Zhang (HKU), M. J. Liu, X. P. Xu, C. C. Jin, Z. X. Ling, W. M. Yuan, Y. Liu, C. Zhang, W. Chen, H. Q. Cheng, C. Z. Cui, D. W. Fan, H. B. Hu, J. W. Hu, M. H. Huang, D. Y. Li, H. Y. Liu, Z. Z. Lv, T. Y. Lian, X. Mao, H. W. Pan, X. Pan, H. Sun, W. X. Wang, Y. L. Wang, Y. F. Xu, H. N. Yang, M. Zhang, W. D. Zhang, W. J. Zhang, Z. Zhang, D. H. Zhao (NAOC, CAS), Y. Chen, S. M. Jia, W. W. Cui, D. W. Han, C. K. Li, L. M. Song, X. F. Zhao, J. Zhang, S. N. Zhang (IHEP, CAS), E. Kuulkers, A. Santovincenzo (ESA), P. O'Brien (Univ. of Leicester), K. Nandra, A. Rau (MPE), B. Cordier (CEA) on behalf of the Einstein Probe team

We report on the detection of two fast X-ray transients designated EP240918b and EP240918c, respectively, by the Einstein Probe (EP) mission. 

EP240918b was detected by EP-WXT at 2024-09-18T15:40:00 (UTC). The WXT position is R.A. = 258.66 deg, DEC = 66.739 deg (J2000) with an uncertainty of about 2.9 arcmin (radius, 90% C.L. statistical and systematic). The light curve of the transient observed by the WXT lasts around 200 seconds. The absorbed average flux in 0.5-4 keV is 2.6(+4.1/-1.2) x 10^(-10) erg/s/cm^2 with a freely fitted column density value of 3.2(+6.2/-3.2) x 10^21 cm^-2 and the photon index of 1.5(+1.8/-1.3). We note that a star, TYC 4210-373-1, is located within 3 arcmin around the source position. 
 
EP240918c was detected by EP-WXT at 2024-09-18T18:06:47 (UTC). The WXT position is R.A.=281.338 deg, DEC=-13.167 deg (J2000) with an uncertainty of about 2.3 arcmin (radius, 90% C.L. statistical and systematic), corresponding to Galactic l, b = 20.503, -4.638. The WXT light curve lasts around 100 seconds with a peak flux of ~ 5.8 x 10^(-9) 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 1.6(+0.8/-0.8) (with column density fixed at the Galactic value 3.7 x 10^21 cm^-2). The derived average unabsorbed 0.5-4 keV flux is 1.5(+0.6/-0.2) x 10^(-9) 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 position.

Following the WXT detections, we performed two follow-up observations with the Follow-up X-ray Telescope (FXT) on board EP. Further follow-up observations are encouraged to identify the nature of this X-ray transient.  

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).