Einstein Probe detection of an X-ray transient EP240904a

ATel #16805; H. Q. Cheng, S. X. Wen (NAOC, CAS), Y. Wang (PMO, CAS), Z. Yan (YNAO, CAS), C. C. Jin, H. Sun, W. Yuan, Z. X. Ling, Y. Liu, C. Zhang, W. Chen, C. Z. Cui, D. W. Fan, H. B. Hu, J. W. Hu, M. H. Huang, D. Y. Li, T. Y. Lian, H. Y. Liu, M. J. Liu, Z. Z. Lv, X. Mao, H. W. Pan, X. Pan, W. X. Wang, Y. L. Wang, Q. Y. Wu, X. P. Xu, 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, H. Feng, 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
on 7 Sep 2024; 09:13 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Yuan Liu (liuyuan@bao.ac.cn)

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We report on the detection of an X-ray transient designated EP240904a (EP J182730.0-095633), by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The transient was first detected by WXT at 2024-09-04T17:20:40 (UTC). The average 0.5-4 keV spectrum can be fitted by an absorbed power law (with the column density fixed at the Galactic value of 1.32 x 10^22 cm^-2) with a photon index of 0.6(-1.3, +1.1). The derived unabsorbed flux in 0.5-4 keV is 4.2(-1.6, +1.8) x 10^(-11) erg/s/cm^2.

We conducted a follow-up observation with the Follow-up X-ray telescope (FXT) onboard EP which started at 2024-09-06T10:23:39 (UTC). The observation, with an exposure time of 5.9 ks, shows an uncatalogued X-ray source located at R.A. = 276.8750 deg, DEC = -9.9426 deg (J2000) with an uncertainty of 10 arcsec in radius (90% C.L. statistical and systematic). The average 0.5-10 keV spectrum can be fitted by an absorbed power law with a photon index of 1.95(+/-0.01) and a free column density of 3.0(+/-0.1) x 10^22 cm^-2, giving an average unabsorbed 0.5-10 keV flux of 2.1(+/-0.1) x 10^(-10) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.

No known X-ray sources were found within the error circle of FXT position. Continuous monitoring of this new transient will be performed with EP-FXT in the following weeks. Swift and NICER observations have also been requested. More multi-wavelength follow-up observations are encouraged to explore this new source.

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.