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GCN Circular 36270

LIGO/Virgo/KAGRA S240422ed: X-ray Upper Limits from EP-WXT for the Three Potential Electromagnetic Counterparts Reported So Far
2024-04-24T03:44:51Z (3 months ago)
EP Team at NAOC/CAS <>
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Y. L. Wang, W. J. Zhang, H. N. Yang, X. Pan, X. P. Xu, C. C. Jin, D. Y. Li, H. Sun, Y. Liu, Z. X. Ling, C. Zhang, W. Chen, H. Q. Cheng, C. Z. Cui, D. W. Fan, H. B. Hu, J. W. Hu, M. H. Huang, J. J. Jin, T. Y. Lian, H. Y. Liu, M. J. Liu, Z. Z. Lv, X. Mao, H. W. Pan, W. X. Wang, S. X. Wen, Q. Y. Wu, Y. F. Xu, W. Yuan, M. Zhang, W. D. Zhang, Z. Zhang, D. H. Zhao (NAOC, CAS), Q. C. Shui (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 X-ray upper limits from the ongoing observations of the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, for the three potential electromagnetic counterparts of the LIGO/Virgo/KAGRA candidate NSBH event S240422ed (The LIGO-Virgo-KAGRA Collaboration, GCN 36236), as reported in two previous GCN/LVC COUNTERPART NOTICEs (Swift-XRT #1, #2) and in GCN 36267 (Magellan #1).

Through our preliminary analysis of the WXT data, we found no significant X-ray counterpart at the position of these sources. The upper limits in 0.5-4 keV at the 90% confidence level are derived from the accumulated exposure from 2024-04-23 00:37:26 (UTC) to 2024-04-23 23:48:09 (UTC). 

The flux upper limits at the 90% confidence level are detailed as follows. 
Source         |  R.A. (J2000)    |   Dec. (J2000)   |  Exposure (s)  | Upper limit (erg/s/cm2) 
Swift-XRT #1   |     120.3245     |    -28.1765     |     43000      |  4.4 x 10^-12
Swift-XRT #2   |     126.2283     |    -23.1542     |     27000      |  5.6 x 10^-12
Magellan #1    |     119.0233     |    -22.8983     |     37000      |  4.7 x 10^-12

For the potential optical counterpart found by Magellan (GCN 36267), we have triggered a quick follow-up observation with the Follow-up X-ray Telescope (FXT) of EP. Further results will be reported after the observation is completed.

The above observations were 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.
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