The Swift follow-up of EP260531a refines its sky position

ATel #18099; Lara Sidoli (INAF-IASF Milano, Italy)
on 9 Oct 2026; 14:57 UT
Credential Certification: Lara Sidoli (sidoli@lambrate.inaf.it)

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A Swift ToO observation of the X-ray transient EP260531a (ATel #17828) was performed on 28 September 2026 (net exposure time of 1.6 ks) to refine its sky position.

Only one source is detected in the entire Swift/XRT field of view (0.3-10 keV), and it is positioned within the error region of EP260531a (ATel #17833).

The source count rate is 0.95+/-0.03 count/s (0.3-10 keV; XRT/PC mode), and appears piled-up. Using the UK Swift Science Data Centre pipeline, made publicly available by the University of Leicester (Evans et al. 2020), the enhanced XRT position of the source is R.A. (J2000) = 265.71165, dec= -31.19918, with an error radius of 2.6 arcsec (90% confidence level).

This refined position excludes both NIR candidate counterparts proposed in ATel #17840.

The source light curve appears variable on timescales of hundred seconds. The XRT spectrum is quite absorbed and when fitted with a power-law model, the hydrogen column density is 5.9 (+1.0, -0.9)e22 cm-2, the power-law photon index is 5.3 (+0.7, -0.6) and the observed flux is 4e-11 erg/s/cm^2 (0.3-10.0 keV). The source is approximately 5 times fainter than when observed by EP-FXT on 4 June 2026 (ATel #17833), and with a similar spectrum.