GCN Circular 36867

W. J. Zhang, W. X. Wang (NAOC, CAS), Y. C. Fu (BNU), C. C. Jin, W. 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, Z. X. Ling, H. Y. Liu, M. J. Liu, Z. Z. Lv, T. Y. Lian, X. Mao, H. W. Pan, X. Pan, H. Sun, Y. L. Wang, S. X. Wen, Q. Y. Wu, X. P. Xu, Y. F. Xu, H. N. Yang, M. Zhang, W. D. Zhang, Z. Zhang, D. H. Zhao (NAOC, CAS), J. Guan, C. K. Li, Y. Chen, S. M. Jia, W. W. Cui, H. Feng, D. W. Han, W. Li, C. Z. Liu, F. J. Lu, L. M. Song, J. Wang, J. J. Xu, J. Zhang, S. N. Zhang, H. S. Zhao, X. F. Zhao (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

The candidate X-ray transient (RA=336.513, DEC=-15.302) triggered by the Wide-field X-ray Telescope (WXT) on board Einstein Probe (EP) at 17:12:56 UT on July 14th, 2024 is likely a stellar flare associated with the high proper motion star LP 820-19.

Upon the detection of the event by WXT, the onboard processing and triggering unit triggered autonamous follow-up observation by the Follow-up X-ray Telescope (FXT) on board EP. The observation started at 2024-07-14T17:14:44, about 2 minutes after the WXT detection, with a net exposure time of 3.0 ks.

Within the error circle of WXT, an X-ray source was clearly detected at R.A. = 336.5083 deg, DEC = -15.3029 deg, with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic). The FXT position is 8.5 arcsec away from a known optical source. The source is likely associated with a possible high proper motion star, LP 820-19, approximately 24.4 pc in distance. The spectrum can be well fitted with a model comprising two absorbed apec components with a temperature of 0.93 (+/-0.03) keV and 4.1 (+/- 0.2) keV, respectively. The column density is 9.0 (-0.6/+0.7) x 10^20 cm^-2. The unabsorbed flux of the FXT source is 3.6 (+/- 0.3) x 10^-10 erg/s/cm^2 in the 0.5-10 keV range. If the FXT source is indeed associated with the high proper motion star, the flare reached a peak luminosity of 4.0 x 10^31 erg/s in the 0.5-10 keV range. (The quoted errors of the parameters derived above are at the 90% C.L.). We thus suggest that the X-ray flare detected by WXT is likely from the high proper motion star.

We have proposed EP and Swift target-of-opportunity observations. Further multi-wavelength follow-up observations are encouraged to monitor the evolution of this X-ray transient.

The above observation was made with the EP-FXT instrument. 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.