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GCN Circular 36282

Subject
LIGO/Virgo/KAGRA S240422ed: X-ray Upper Limits from EP-WXT for the Potential Electromagnetic Counterparts Reported by Swift and DECam
Date
2024-04-24T13:14:11Z (3 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
H. N. Yang, Y. L. Wang, W. J. Zhang, 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, 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 potential electromagnetic counterparts reported by Swift (Evans et al. GCN 36278) and DECam (Hu et al. GCN 36273). 

Through our preliminary analysis of the WXT data, we found no significant X-ray counterparts at the positions of these sources, except for AT 2024hdq which is associated with a known Seyfert I galaxy LEDA 86073. The detected flux of this Seyfert by EP-WXT is around 2 x 10^-12 erg/s/cm2, consistent with the flux given in eRASS1. Upper limits for the other sources 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       |   Dec.    |  Exposure (s) | Upper limit (erg/s/cm2) 
----------------------------------------------------------------------------------
AT 2024hdk     | 130.625724 | -15.59953 |     20735     |  6.4 x 10^-12
AT 2024hdn     | 127.302473 | -21.50639 |     27013     |  5.6 x 10^-12
AT 2024hdm     | 126.65801  | -20.83375 |     26785     |  5.6 x 10^-12
AT 2024hdl     | 125.472700 | -30.10220 |     39700     |  4.6 x 10^-12
AT 2024hdo     | 120.866797 | -26.01212 |     50042     |  4.1 x 10^-12
AT 2024hdr     | 120.804560 | -24.33268 |     39477     |  4.6 x 10^-12
AT 2024hdp     | 120.543455 | -27.25787 |     42801     |  4.4 x 10^-12
S240422ed_X101 | 122.3954   | -24.3272  |     44192     |  4.4 x 10^-12
S240422ed_X190 | 121.2770   | -26.1203  |     48919     |  4.1 x 10^-12
----------------------------------------------------------------------------------
Note: the X-ray upper limit from EP-WXT for another potential electromagnetic counterpart of Swift, i.e. S240422ed_X61 has been given in our previous report (GCN 36270). 

Follow-up observations with the Follow-up X-ray Telescope (FXT) of EP for the potential optical counterparts found by DECam have been carried out. For the two potential electromagnetic counterparts of Swift (S240422ed_X101, S240422ed_X190), we have also triggered quick follow-up observations with EP-FXT. Further results will be reported after the data analysis 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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