TITLE: GCN CIRCULAR NUMBER: 39834 SUBJECT: FRB 20250316A: detection of a candidate associated X-ray source EP J120944.2+585060 by Einstein Probe DATE: 25/03/22 15:11:56 GMT FROM: EP Team at NAOC/CAS ep_ta@bao.ac.cn

H. Sun, H. Q. Cheng, D. Y. Li, H. Y. Liu, C. C. Jin, Z. X. Ling, W. D. Zhang, Y. Liu (NAO, CAS), B. Zhang (UNLV), D. F. Hu, Y. Li, J. J. Geng, X. F. Wu (PMO, CAS), J. W. Hu, H. W. Pan, C. Zhang, L. Chen, S. Q. Jiang, Y. J. Song, T. Zhao (NAO, CAS), Y. Chen, C. K. Li, J. Guan, S. M. Jia, H. S. Zhao, J. Zhang, M. Y. Ge, W. W. Cui, H. Feng, W. Li, C. Z. Liu, F. J. Lu, L. M. Song, J. Wang, J. J. Xu, D. W. Han, S. N. Zhang, X. F. Zhao, Q. C. Zhao, Z. H. Yang, Q. C. Shui (IHEP, CAS), Q. C. Liu (THU), L. Piro (INAF), V. Burwitz, P. Friedrich, N. Meidinger, K. Nandra, A. Rau (MPE), J.-U. Ness, A. Santovincenzo (ESA), Nanda Rea (ICE-CSIC), P. O'Brien (Univ. of Leicester), B. Cordier (CEA), W. Yuan (NAO, CAS) report on behalf of the Einstein Probe team:

We observed the field of the bright fast radio burst FRB 20250316A (Ng, ATel #17081) with the Follow-up X-ray Telescope (FXT) on board the Einstein Probe (EP) mission. A total of four observations were conducted. In the first observation, which began at 2025-03-17T16:57:29 with an exposure time of 2.9 ks, no X-ray sources were detected within 3 arcmin of the CHIME position. In the second observation, starting at 2025-03-18T15:23:50 with also 2.9 ks exposure, FXT detected a weak, uncatalogued X-ray source, designated as EP J120944.2+585060. The position of this source is consistent with the refined localization provided by CHIME (Leung, ATel #17086). The unabsorbed X-ray flux is 3.4 x 10^-14 erg/cm2/s in the 0.5 - 10 keV band.

In the third observation starting at 2025-03-21T05:50:01 with an exposure of 5.8 ks, the X-ray source was clearly detected at a flux consistent with the previous level. The refined source position is RA = 182.4341 deg, Dec = 58.8499 deg (J2000) with an uncertainty of 10 arcsec in radius (90% C.L. statistical and systematic). This position remains consistent with the CHIME localization, suggestive of EP J120944.2+585060 being likely associated with the source of FRB 20250316A. Preliminary analysis of the forth observation, taken 11 hours after the third one, shows a marginal trend of decrease of the source flux, though the the telemetry data received is not complete yet. Further analysis is ongoing. A Chandra DDT observation has been requested.

If EP J120944.2+585060 is indeed associated with FRB 20250316A and located near the candidate host galaxy NGC 4141, which has a redshift of 0.0067 (Connor, ATel #17091), the inferred X-ray luminosity is approximately 3 x 10^39 erg/s in the 0.5 - 10 keV band during the EP observations.

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.

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