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GCN Circular 36313

LIGO/Virgo/KAGRA S240422ed: EP-FXT detection of a candidate counterpart EP240426a
2024-04-26T11:13:02Z (3 months ago)
EP Team at NAOC/CAS <>
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H. Sun, D. Y. Li (NAOC, CAS), Y. Chen, J. Guan, C. K. Li, 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 (IHEP, CAS), C. C. Jin, J. W. Hu, M. Zhang, H. Q. Cheng, H. Y. Liu, Y. Liu, Z. X. Ling, C. Zhang, M. H. Huang, H. W. Pan, Y. F. Xu, W. Yuan, W. D. Zhang (NAOC, CAS), Y. C. Fu (BNU), B. Zhang (UNLV), V. Burwitz, P. Friedrich, N. Meidinger, K. Nandra, A. Rau (MPE), E. Kuulkers, A. Santovincenzo (ESA), P. O'Brien (Univ. of Leicester), B. Cordier (CEA), on behalf of the Einstein Probe team 

We report on the detection of a possible X-ray counterpart candidate of S240422ed (The LIGO-Virgo-KAGRA Collaboration, GCN 36236), designated EP240426a, by the Follow-up X-ray telescope (FXT) on board the Einstein Probe (EP) mission. This source was found in a series of snapshots of EP-FXT targeting galaxies within the probable location of S240422ed. The observation started at 2024-04-26T04:23:21 (UTC) with an exposure time of 534 seconds.

The source is located within the 90% credible region of the updated LVK sky localization (The LIGO-Virgo-KAGRA Collaboration, GCN 36240). The position is R.A. = 121.8567 deg, Dec. = -29.4609 (R.A. = 08:07:25.61, Dec. = -29:27:39.2) with an uncertainty of 10" (90% C.L. statistical and systematic). This position is 5.5 arcsec away from the galaxy 2MASX J08072584-2927344 at a luminosity distance of 188 Mpc. The absorption-corrected flux in 0.5-10 keV is about 9.2 x 10 ^-13 erg/s/cm2 assuming a power-law spectrum with a photon index of 2.0 with Galactic absorption. 

No previously known X-ray sources are found within the error circle around this position. A flux upper limit set by the eRASS1 survey is 1.2 x 10^-13 erg/s/cm2 (99.9% C.L.) in 0.2-5.0 keV .

For any questions concerning this source please contact by email the Transient Advocates of EP240426a, H. Sun ( and D. Y. Li ( from the EP science center. Please note that EP-FXT is currently undergoing in-flight calibration, and the derived source parameters may be subject to larger uncertainties than quoted here; 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. 
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