GCN Circular 36277

W. J. Zhang, Y. L. Wang, 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), B. Zhang (UNLV), 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 derived from early observations with the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission for the sky region of the  LIGO/Virgo/KAGRA candidate NSBH event S240422ed (The LIGO-Virgo-KAGRA Collaboration, GCN 36236
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). 

A real-time EP-WXT observation serendipitously covered ~90 square degrees in the 90% credible region of the updated LVK sky localization (The LIGO-Virgo-KAGRA Collaboration, GCN 36240
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) from 2024-04-22 21:21:23 UTC with an exposure time of 5900 seconds, 830 s before the trigger time of the gravitational wave (GW) event (T0). However, this observation was interrupted by the SAA passage from 2024-04-22 21:31:23 UTC to 2024-04-22 22:00:00 UTC, so only the data within [T0-830 s, T0-230 s] (Obs-1a) and [T0+1487 s, T0+36202 s] (Obs-1b) are useful. No significant X-ray counterpart was detected in these good time intervals. Flux upper limits in 0.5-4 keV at the 90% confidence level are derived from good exposure time before and after the interruption of the observation.

During the above observation, an EP Target-of-Opportunity observation (Obs-2) was triggered on the LVK sky localization region, with the WXT field-of-view covered almost all of the 90% credible region of GW localization. Through our sebsequent analysis of the WXT data, no significant X-ray counterparts were found. A flux upper limit in 0.5-4 keV at the 90% confidence level is derived from this observation.

The flux upper limits at the 90% confidence level are detailed as follows. 
No.     |  Start time (UTC)   |   End time (UTC) |  Effective Exposure (s)  | Upper limit (erg/s/cm2) 
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Obs-1a  | 2024-04-22 21:21:23 | 2024-04-22 21:31:23 |    600   |  4.1 x 10^-11
Obs-1b  | 2024-04-22 22:00:00 | 2024-04-23 07:38:35 |    5300  |  1.3 x 10^-11
Obs-2   | 2024-04-23 00:37:26 | 2024-04-23 05:24:11 |    5100  |  1.3 x 10^-11

EP is currently observing a list of galaxies within the 90% GW region with its Follow-up X-ray Telescope (FXT). The WXT field-of-view is covering a large fraction of this GW region simultaneously. The monitoring campaign and subsequent data analysis are ongoing. Further results will be reported later.

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.