GCN Circular 36598

C. Y. Wang (THU), H. N. Yang (NAOC, CAS), S. X. Wen (NAOC, CAS), W. X. Wang (NAOC, CAS), W. Yuan, Z. X. Ling, Y. Liu, C. Zhang, C. C. Jin, H. Q. Cheng, W. Chen, C. Z. Cui, D. W. Fan, H. B. Hu, J. W. Hu, M. H. Huang, H. Y. Liu, M. J. Liu, Z. Z. Lv, T. Y. Lian, X. Mao, H. W. Pan, H. Sun, 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, 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

The candidate X-ray transient (RA=336.13, DEC=-58.429) triggered by the Wide-field X-ray Telescope (WXT) on board Einstein Probe (EP) at 17:15:42.949 UT on June 1st, 2024 is likely a stellar flare associated with the Eruptive Variable UPM J2224-5826.

Upon the detection of the event by WXT, the onboard processing and triggering unit triggered automated follow-up observation by the Follow-up X-ray Telescope (FXT) on board EP. The observation started at 2024-06-01T17:17:40, 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. = 336.101 deg, DEC = -58.437 deg, with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic). The FXT position is 3.3 arcsec away from a known X-ray source, 1eRASS J222424.3-582612, which has an archival flux of 2.48 x 10^-13 erg/s/cm^2 in the 0.2-2.3 keV band. The source is likely associated with a possible M-type dwarf star, UPM J2224-5826, approximately 16 pc in distance. The spectrum can be well fitted with a model comprising two apec components with a temperature of 0.89 (-0.07/+0.05) keV and 2.9 (+/- 0.3) keV, respectively. The flux of the FXT source is 3.0 (+/- 0.1) x 10^-11 erg/s/cm^2 in the 0.5-10 keV range, which is two orders of magnitude higher than the eROSITA flux. If the FXT source is indeed associated with the M star, the flare reached a peak luminosity of 7.6 x 10^29 erg/s in the 0.5-4.0 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 M dwarf star.   

Please note that EP-FXT is currently undergoing in-flight calibration. The derived source parameters may be subject to larger uncertainties, so please use them with caution. 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.