EP240417a

J. W. Hu, W. Chen, S. Q. Jiang, C. C. Jin, Y. Liu, Z. X. Ling, C. Zhang, H. Q. Cheng, C. Z. Cui, D. W. Fan, H. B. Hu, M. H. Huang, D. Y. Li, T. Y. Lian, H. Y. Liu, M. J. 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, W. Yuan, M. Zhang, W. D. Zhang, W. J. Zhang, Z. Zhang, D. H. Zhao (NAOC, 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 EP240417a at 2024-04-17T15:12:33 (UTC) by the Wide-field X-ray Telescope (WXT) onboard the Einstein Probe (EP) mission during a calibration observation. The position of the source is R.A. = 177.442 deg, DEC = -15.438 deg (J2000) with an uncertainty of 3 arcmin (radius, 90% C.L. statistical and systematic). The flare lasted for at least 1500 seconds, with the tail of the fading light curve undetected due to the interruption of the observation. The peak flux reached ~3 x 10^-10 erg/s/cm^2 in the 0.5-4.0 keV band. The averaged 0.5-4.0 keV spectrum can be well fitted with a single power law of photon index 1.15(-0.26/+0.27), and the best-fit absorption column density is consistent with 0. The derived average 0.5-4.0 keV flux is 9.8(-0.8/+0.8) x 10^-11 erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.
 
No previously known bright X-ray sources are found within the error circle around the source position. The derived source parameters may be subject to larger uncertainties than those quoted here since the in-orbit calibration of the instrument is still in progress.

Further follow-up observations are encouraged to identify the nature of this X-ray transient.

The above observation was made with EP-WXT 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.

Tian-Rui Sun, Jin-Jun Geng, Jian Chen, Yan-Long Hua and Lei Hu report on behalf of the YAHPT team:

Following the detection of EP240417a by Einstein Probe (Hu et al., GCN 36161
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 ), 
we use the Yaoan High Precision Telescope at Yaoan Astronomy Observation Station (Yunnan province, China) to search and follow up.

We observed the target position with 300s exposure in Rc-band starting from 2024-04-18T16:35:20.400 about.
We took 5 images with total exposure time of 1500 seconds from 16:35:20 to 16:55:30. 
We did not detect any possible optical counterpart up to Rc = 20.7 within the 3-arcmin radius.

We used the USNO B1.0 catalog (R2mag) as the magnitude reference for calibration.

J. Quirola-Vásquez (Radboud Univ.), F. E. Bauer (PUC), P.G. Jonker (Radboud Univ.), A. J. Levan (Radboud Univ. & Warwick Univ.), D. B. Malesani (DAWN/NBI and Radboud Univ.) on behalf of a larger collaboration:

We observed the field of the Einstein Probe fast X-ray transient EP240417a (Hu et al., GCN 36161
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 ) with the SOAR telescope of Cerro Pachón observatory on 2024-04-20 starting (UT) 00:22:04.167, i.e., ~2.4 days after the X-ray trigger. We took 2 images using the Goodman Spectrograph RedCam in imaging mode, and  the i-SDSS filter, with a total exposure time of 1100 seconds. We did not detect an optical counterpart in the field of EP240417a (R.A. = 177.442 deg and DEC = -15.438 deg, with an uncertainty of 3 arcmin) down to the limiting magnitude of i~23 AB mag. None new detection or >2sigma variability regarding the Legacy catalog. We used the PanSTARRS catalog as the magnitude reference for calibration.

We acknowledge the excellent support from the SOAR staff.