EP-FXT and Swift/XRT observations of EP250623a/EP J171159.4-333253: likely a new neutron star X-ray binary with type I bursts

ATel #17255; Jiahua Wu (GZHU), K. R. Ni (CCNU), W. F. Wen (SZTU), D. Y. Li (NAO, CAS), J. W. Hu (NAO, CAS), Y. L. Wang (NAO, CAS; ICE, CSIC-IEEC), B. T. Wang (YNO, CAS), P. Y. Han (HUST), Z. X. Ling (NAO, CAS), H. W. Pan (NAO, CAS), F. Coti Zelati, A. Marino, N. Rea (ICE, CSIC-IEEC), S. Guillot (IRAP), H. Feng, L. Tao (IHEP, CAS), and W. Yuan (NAO, CAS), on behalf of the EP team
on 28 Jun 2025; 11:55 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Yuan Liu (liuyuan@bao.ac.cn)

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We report on monitoring observations of EP250623a/EP J171159.4-333253 by the Follow-up X-ray Telescope (FXT) on board EP and the Swift/XRT. The source was first detected with the Wide-field X-ray Telescope (WXT) on EP at 2025-06-23T18:47:06 (UTC) and initially followed up with FXT at 2025-06-25T13:58:42 UTC (K.-R. Ni et al., see ATel #17247). Based on the FXT detection, we renamed the source as EP J171159.4-333253. This replaces the provisional designation of EP J171203.4-333221.5 based on the EP-WXT detection, which was used in ATel #17247 (K.-R. Ni et al.).

An additional FXT observation was performed starting at 2025-06-26T13:58:10 (UTC), with an exposure time of 6000 s. The X-ray spectrum can be modeled reasonably well by an absorbed powerlaw plus a blackbody, with a photon index of 1.40 (-0.25, +0.39) and a blackbody temperature of 1.75 (-0.75, +0.45) keV. The absorption column density is 3.68 (+/- 0.60) e21 cm^-2, consistent with the previous measurements. The derived unabsorbed flux in 0.5-10 keV is 1.50 (+/-0.06) e-10 ergs/s/cm^2. During the observation, the lightcurve shows another flaring event with a duration of ~200 seconds, similar to the initial flare reported in ATel #17247 (K.-R. Ni et al.), which is possibly a type I burst. Notably, the FXT light curve reveals a deep eclipse-like feature with a duration of about ~700 seconds. We tend to suggest EP250623a/EP J171159.4-333253 likely to be an eclipsing X-ray binary system hosting a neutron star.

We also requested a Swift ToO observation (obsID 00019894001) starting at 2025-06-26T16:46:56 UTC (exposure 1049 s). The source was also detected with XRT at a refined position of R.A. = 257.99740 deg, Dec. = -33.54763 deg (with an uncertainty of 2.6 arcsec in radius, 90% c.l.), which is consistent with the EP-FXT position within the uncertainties. The XRT spectrum can be fitted with an absorbed powerlaw model, with a photon index of 1.12 (+/- 0.15) and an absorption column density of 3.99 (+/- 0.10) e21 cm^-2. The derived unabsorbed flux in 0.5-10 keV is 1.61 (+/-0.11) e-10 ergs/s/cm^2. Adding a blackbody component does not improve the fit statistically. The uncertainties of the fitted parameters quoted above are at the 90% confidence level.

We analyzed the Swift-UVOT data. No new sources are found in the U-band within the XRT positional error circle, setting a 3-sigma upper limit of 20.82 mag (Vega system). We note that there is a faint optical object on the B-band image within the XRT positional error circle, which is spatially coincident with a known source, Gaia DR3 5979701138818118016, although a chance coincidence of this source with EP250623a cannot be ruled out.

More monitoring observations on this source are planned with the EP-FXT. Further multiwavelength observations are encouraged to identify the nature of this transient. We thank the Swift team for quick response and making the ToO observations possible. 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. The contact TA of this source is Jiahua Wu, please contact him via the email jhwu@e.gzhu.edu.cn if needed.