EP-WXT detection of an X-ray transient EP240809a and Swift follow-up observation

ATel #16765; H. Y. Liu, H. N. Yang, D. Y. Li, D. H. Zhao, W. Yuan, H. Sun, Y. Liu, C. C. Jin, Z. X. Ling, C. Zhang, H. Q. Cheng, W. Chen, C. Z. Cui, D. W. Fan, H. B. Hu, J. W. Hu, M. H. Huang, M. J. Liu, Z. Z. Lv, T. Y. Lian, X. Mao, H. W. Pan, X. Pan, W. X. Wang, Y. L. Wang, Q. Y. Wu, X. P. Xu, Y. F. Xu, M. Zhang, W. D. Zhang, W. J. Zhang, Z. Zhang (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 11 Aug 2024; 15:31 UT
Credential Certification: Yuan Liu (liuyuan@bao.ac.cn)

Tweet

We report on the detection of an X-ray transient designated EP240809a, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The transient was detected during a long observation which started at 2024-08-09T16:53:08 (UTC) with an exposure time of about 40 ks. The WXT position of EP240809a is R.A.= 268.259 deg, DEC = -45.868 deg (J2000) with an uncertainty of 2.2 arcmin (radius, 90% C.L. statistical and systematic). The WXT lightcurve does not show significant variability. The average 0.5-4 keV spectrum can be fitted by an absorbed power law with a photon index of 1.6(-0.2, +0.2) with the column density fixed at the Galactic value 1.62 x 10^21 cm^-2. The derived unabsorbed flux in 0.5-4 keV is 5.3(-0.6, +0.7) x 10^(-11) erg/s/cm^2.

We requested a Swift ToO observation which started at 2024-08-10T23:58:56 (UTC). A bright source was clearly detected within the WXT position uncertainty at R.A. = 268.2688 deg, Dec=-45.8635 deg by XRT, with an estimated uncertainty of 3.7 arcsec (radius, 90% C.L. statistical and systematic). The XRT spectrum, which can be fitted with an absorbed power law with a photon index of 1.87(-0.14, +0.14) modified by Galactic absorption, gives an unabsorbed 0.3-10 keV flux of 1.4(-0.1, +0.1) x 10^(-11) erg/s/cm^2. Meanwhile, an uncatalogued source was likely detected by UVOT (u, uvm2, and uvw2 band) at the location of R.A. = 268.26769 deg, DEC = -45.86380 deg with an uncertainty of 0.48 arcsec (radius, 90% C.L. statistical and systematic). The uncertainties are at the 90% confidence level for the above parameters.

We thank the Swift team for performing the ToO observation. Follow-up observations are encouraged to identify the nature of this transient.

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.