TITLE: GCN CIRCULAR NUMBER: 39545 SUBJECT: LIGO/Virgo/KAGRA S250206dm: early X-ray upper limits from Einstein Probe WXT and preliminary results of FXT observations DATE: 25/03/02 03:55:41 GMT FROM: EP Team at NAOC/CAS ep_ta@bao.ac.cn

D. Y. Li, J. W. Hu, Q. Y. Wu, M. J. Liu, R. D. Liang, H. Sun (NAOC, CAS), A. Li (BNU), C. Q. Shui (IHEP, CAS), M. H. Zhang, X. P. Xu, W. X. Wang, H. Q. Cheng, H. N. Yang, D. H. Zhao, X. Pan, Y. F. Xu, M. Zhang (NAOC, CAS), X. Y. Zhou (PRIC), Y. C. Fu (BNU), S. F. Zhu (CSTU), Y. J. Zhang (THU), H. L. Peng (NJNU), G. Y. Zhao (SYU), C. Zhou (HUST), T. C. Zheng (PMO, 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), Y. Liu, W. D. Zhang, Z. X. Ling, C. C. Jin, H. W. Pan, C. Zhang, W. Yuan (NAOC), B. Zhang (UNLV), L. Piro (INAF), 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) report on behalf of the Einstein Probe team:

We report on preliminary results of the observations targeting the sky regions of the LIGO/Virgo/KAGRA NS-BH gravitational-wave event S250206dm (The LIGO-Virgo-KAGRA Collaboration, GCN 39175) to search for any potentially associated X-ray source with the Einstein Probe (EP). Two rounds of observations were carried out with the Wide-field X-ray Telescope (WXT) and Follow-up X-ray Telescope (FXT) on board EP between Feb. 7th and 18th 2025. No promising X-ray transient candidates have been found. The information on the sky coverage and observations can be found at the web page https://ep.bao.ac.cn/ep/cms/article/view?id=185.

First, two early WXT observations were performed, starting at 2025-02-07T02:55:12.000Z and 2025-02-07T03:40:22.000Z, about 4.5 hours and 6 hours after the GW event, respectively, each with an exposure time of 2400s. These observations covered 302 square degrees within (55% of) the 90% credible region of the updated LVK sky localization (The LIGO-Virgo-KAGRA Collaboration, GCN 39231). No new X-ray sources were detected. The 0.5-4 keV flux limits over the covered regions are set to be approximately 1.0 x 10^-11 erg/s/cm2 at the 90% confidence level in general.

Then, 100 pointed FXT observations were carried out during Feb. 7th and 8th, each with a 300s-exposure, targeting galaxies located within the 90% credible region and the distance uncertainty range of S250206dm. The targets are among the top 500 galaxies that were selected from the GLADE+ catalog and prioritized based on their location, mass, and visibility to FXT. These observations covered 32 (24) square degree in the 90% (50%) credible region, setting 0.5-10 keV flux limits of generally 3e-13 erg/s/cm2 at the 90% confidence level.

From Feb. 11th to 18th, 2025, the second round of observations consisted of tiling observations with FXT, covering 34 square degrees in (89% of) the 50% credible region of S250206dm. There were 59 observations performed, each with an exposure of 2000s, setting the average flux limits of approximately 5e-14 erg/s/cm2 at the 90% confidence level.

We searched for transient or variable candidates in the FXT data which are apparently associated with the galaxies in the NED-LVS sample in the credible region and distance range of S250206dm (Cook et al. GCN 39235) within a radius of 30 arcsec. Two uncatalogued sources were found to exhibit significant flux decay from the FXT observations, whose details are listed in the table below. The fluxes are in the 0.5-10 keV band and estimated from spectral fitting with an absorbed powerlaw model. EPF J022731.1+551813 was not detected in the second observation, and an upper limit at the 90% confidence level is provided. We note that, however, a stellar flare origin of these two sources cannot be ruled out. ------------------------------------------------------------------------------------------------------ | Source Name | Obs_Time (UTC) | RA | DEC | Flux(erg/s/cm2) | Flux_err | ------------------------------------------------------------------------------------------------------ |EPF J022731.1+551813| 2025-02-13 08:35:07 | 36.8797 | 55.3037 | 5.28e-13 | 2.27e-14 | | | 2025-02-15 13:24:47 | | | <7.0e-14 | | ------------------------------------------------------------------------------------------------------ |EPF J022842.7+542051| 2025-02-08 07:59:43 | 37.1780 | 54.3475 | 6.61e-13 | 1.94e-14 | | | 2025-02-11 18:44:27 | 37.1839 | 54.3466 | 1.09e-13 | 4.27e-14 | | | 2025-02-15 10:12:36 | 37.1795 | 54.3469 | 1.07e-13 | 2.81e-14 | ------------------------------------------------------------------------------------------------------

Follow-up observations are encouraged to identify the nature of these two variable sources.

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).

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