------------------------------------------------------------------------------ The Astronomer's Telegram                   http://www.astronomerstelegram.org ============================================================================== ATEL #16546     ATEL #16546 Title: Einstein Probe detects a highly variable X-ray source EP240309a (EP J115415.8-501810) Author: Z. X. Ling, M. J. Liu, Y. Liu, C. C. Jin, C. Zhang, H. Q. Cheng, 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, Z. Z. Lv, X. Mao, H. W. Pan, X. Pan, H. Sun, 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, W. W. Cui, H. Feng, J. Guan, D. W. Han, S. M. Jia, C. K. Li, 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, Andrea Santovincenzo (ESA), Richard Saxton (EASC), P. O'Brien (Univ. of Leicester), Arne Rau, Kirpal Nandra, P. Friedrich, N. Meidinger, V. Burwitz (MPE), B. Cordier (CEA), Nanda Rea (ICE-CSIC) and W. Yuan (NAOC, CAS), on behalf of the Einstein Probe team Queries: ep_ta@bao.ac.cn Posted: 21 Mar 2024; 08:46 UT Subjects:X-ray, Transient We report the detection of a highly variable X-ray source by the Einstein Probe (EP) mission, designated EP240309a/EP J115415.8-501810. The source was detected during a calibration observation by the Wide-field X-ray Telescope (WXT) on board EP at 2024-03-09T01:44:59 (UTC) at R.A. = 178.57 deg, DEC = -50.29 deg (J2000), with an uncertainty of 2.1 arcmin (radius, 90% C.L. statistical and systematic). The source remained detectable by WXT in subsequent observations until March 16, 2024, with its 0.5-4 keV flux ranging from 5e-12 to 7e-12 erg/s/cm^2. An upper limit to the flux of 9.4e-12 erg/s/cm^2 is set by the latest WXT observation on March 17, 2024. The source was subsequently detected by the Follow-up X-ray Telescope (FXT) on board EP at 2024-03-16T02:00:00 (UTC), with an improved position at R.A. = 178.566, DEC = -50.303 deg (J2000, with an uncertainty of about 10 arcsec). We examined historical observations around this position and found a known X-ray source, XMMSL J115415.6-501758, detected during a slew of XMM-Newton on 2022-01-10 at a flux of 3.5e-12 erg/s/cm^2 in 0.2-2 keV. The separation of the two sources is 12 arcsec, consistent with them being the same source. However, it was not detected in previous observations by XMM-Newton on 2009-01-21 nor by Swift/XRT over various observations between 2011 and 2012. Swift/XRT detected it on 2021-07-20 with a 0.3-10 keV flux of 1.5e-11 erg/s/cm^2. Consistent with the FXT position, eROSITA detected a faint source, 1eRASS J115415.7-501758, in its first six months of the all-sky survey (Merloni et al. 2024). The 0.2-2.3keV flux was 3.0e-14 erg/s/cm^2. Preliminary analysis shows that the FXT spectrum can be fitted by an absorbed power-law with NH = 3.3(-1.3/+2.0)e21 cm-2 and a photon index of 0.95(-0.25/+0.31). The derived 0.3-10 keV flux is 7.4e-12 erg/s/cm^2, slightly higher than that of XMM-Newton (3.5e-12 erg/s/cm^2). These observations suggest that the X-flux varied by at least two orders of magnitude. We note that a bright UV source with a highly variable optical counterpart, Gaia DR3 5370642890382757888 (g magnitude varying from 17 to 14), is located within the 10 arcsec error of the FXT position. This suggests that EP240309a is likely of Galactic origin and potentially a candidate cataclysmic variable (CV). Further follow-up observations are encouraged to identify the nature of this source. The above observations were made by WXT and FXT on board EP during the commissioning phase. 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. ------------------------------------------------------------------------------