EP240703b

J. Q. Peng, Q. C. Shui (IHEP, CAS), C. Y. Dai (NJU), Y. L. Wang, C. C. Jin, Z. X. Ling, W. Yuan, Y. Liu, C. Zhang, H. Q. Cheng, C. Z. Cui, D. W. Fan, H. B. Hu, J. W. Hu, M. H. Huang, D. Y. Li, H. Y. Liu, M. J. Liu, Z. Z. Lv, T. Y. Lian, X. Mao, H. W. Pan, X. Pan, H. Sun, W. X. Wang, Y. L. Wang, S. X. Wen, 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, 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 

We report on the detection of a fast X-ray transient, designated EP240703b, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The transient started at 2024-07-03T05:24:26 (UTC). The position of the source is R.A. = 279.539 deg, DEC = -57.401 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic). The corresponding Galactic coordinates are l = 338.077, b = -20.878.

The transient lasted for approximately 600 seconds and had a peak absorbed flux of ~ 3 x 10^-9 erg/s/cm^2 in the 0.5-4 keV band. The average 0.5-4 keV spectrum can be fitted by an absorbed power law with a column density of 1.4(+1.3/-1.2) x 10^21 cm^-2 and a photon index of 1.5(+0.6/-0.5). The derived average unabsorbed 0.5-4 keV flux is 7.5(+1.3/-1.8) x 10^-10 erg/s/cm^2. If the column density is fixed at the Galactic value of 6.8 x 10^20 cm^-2, the derived photon index is 1.2(+/-0.2) and the average unabsorbed 0.5-4 keV flux is 7.1(+1.1/-1.0) x 10^-10 erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters. No previously known X-ray sources at a similar flux level are found within the 3 arcmin region around the source position. 

We have proposed a Swift target of opportunity observation. Further follow-up observations are encouraged to identify the nature of this X-ray transient. 

The above observation was made with the WXT instrument during the commissioning phase of EP. 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.

S. Tinyanont, R. Anutarawiramkul, P. Butpan (NARIT), X. Liu, S.Q. Jiang, J. An, S.Y. Fu, Z.P. Zhu, D. Xu, Z. Fan, W.X. Li, N.C. Sun, Y.N. Wang (NAOC) report on behalf of a large collaboration:

We observed the field of the fast X-ray transient EP240703b (Peng et al., GCN 36810
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 ) using the 0.7-m telescope of the Thai Robotic Telescope (TRT) network, located at Spring Brook Observatory, Australia. Observations started at 09:09:53 UTC on 2024-07-04, i.e., 1.16 day after the EP trigger, and 5 x 360 s  frames in the R- & I- bands were obtained, respectively.

No uncatalogued optical transient is detected in the stacked image within the 3-arcmin EP/WXT error circle, down to a 5-sigma limiting magnitudes of R ~ 21.3 and I ~ 20.4, calibrated with SkyMapper sources in the field.

W. X. Li, S. J. Xue (NAOC), M. Andrews, J. Farrah, D. A. Howell, M. Newsome, E. Padilla Gonzalez, C. McCully, and G. Terreran (Las Cumbres Observatory), on behalf of a larger collaboration, report:
Following the detection of the fast X-ray transient EP240703b by the Einstein Probe (Peng et al., GCN 36810
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 ), we initiated observations of the fast X-ray transient location starting on July 4 at 6:01 UT (~24 hours after the trigger) in the r band. These observations were conducted using the 1-meter telescope at the Las Cumbres Observatory node located at Cerro Tololo Inter-American Observatory in Chile.
No new optical source was detected in the co-added images within the EP/WXT error box down to ~21.0 mag.
These observations were taken as part of the Global Supernova Project.