TITLE:   GCN CIRCULAR
NUMBER:  36840
SUBJECT: EP240708a: X-ray follow-up observations with EP/FXT and Swift/XRT
DATE:    24/07/10 10:01:58 GMT
FROM:    EP Team at NAOC/CAS <ep_ta@bao.ac.cn>

J. W. Hu, W. Chen, Y. L. Wang, C. C. Jin, Z. X. Ling, Y. Liu, C. Zhang, H. Q. Cheng, C. Z. Cui, D. W. Fan, H. B. Hu, M. H. Huang, D. Y. Li, H. Y. Liu, M. J. Liu, T. Y. Lian, Z.Z. Lv, X. Mao, H. W. Pan, X. Pan, H. Sun, W. X. Wang, Q. Y. Wu, X. P. Xu, Y. F. Xu, H. N. Yang, W. Yuan, M. Zhang, W. D. Zhang, W. J. Zhang, Z. Zhang, D. H. Zhao (NAOC, CAS), Y. Chen, S. M. Jia, 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 

Following the detection of the fast X-ray transient EP240708a (Chen et al., GCN 36838), we performed an observation of EP240708a with the Follow-up X-ray Telescope (FXT) on board the Einstein Probe mission. The observation began at 2024-07-09T03:45:47(UTC), about 4 hours after the EP-WXT trigger. The exposure time is 1603 seconds. A faint source was detected at R.A. = 345.9656 deg, DEC = -22.8428 deg, with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic). The spectrum can be fitted with an absorbed power-law model with a photon index of 1.62(-0.60/+0.65) and the column density fixed at the Galactic value of 2.0 x 10^20 cm^-2. The derived unabsorbed flux in 0.5-10 keV is 2.37(+2.36/-1.08) x 10^-13 erg/s/cm^2 (90% C.L.). 
 
We also triggered a Swift target of opportunity observation (ObsID: 00016707001). The XRT onboard Swift began the observation at 2024-07-09T16:44:14 UTC, about 17 hours after the WXT detection, with an exposure time of 1843 s in the Photon Counting mode. No significant X-ray source was detected within the 3 arcmin region around the WXT position. Assuming an absorbed power-law model with a Galactic column density of 2.0 x 10^20 cm^-2 and a photon index of 1.62, the estimated flux upper limit in 0.3-10 keV at the 90% confidence level is 2.67 x 10^-13 ergs/s/cm^2. 

We thank the Neil Gehrels Swift Observatory team for making the X-ray observation possible. 

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).