Swift follow-up of the X-ray transient EP250916a
ATel #17397; Giulia Illiano (INAF-OAB), Arianna Miraval Zanon (ASI), Boris Sbarufatti (INAF-OAB), Alessandro Papitto (INAF-OAR), Maria Cristina Baglio (INAF-OAB), Caterina Ballocco(INAF-OAR)
on 18 Sep 2025; 15:12 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Giulia Illiano (giulia.illiano@inaf.it)
Subjects: X-ray, Transient
Referred to by ATel #: 17421
On 2025 September 16, the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) detected a new X-ray transient, designated EP250916a (GCN #41841). Subsequent EP observations revealed a rapid brightening of the source (GCN #41861; ATel #17395), confirmed also by SVOM observations (ATel #17396).
Follow-up Swift/XRT observations were carried out on 2025 September 17. A first exposure, obtained in Photon Counting (PC) mode, yielded an enhanced position of R.A.=268.23865 deg, DEC. =-35.32302 deg (J2000), with an associated uncertainty of 2.2 arcsec (90% c.l.) based on a total exposure of 872 s in the final position.
A second observation in Windowed Timing (WT) mode began on 2025 September 17 at 23:08:08 UTC with an exposure time of 1.9 ks. The Swift/XRT spectrum is well fitted with an absorbed power-law model, yielding a hydrogen column density NH = (3.8 ± 0.2) × 1021 cm-2 and a photon index Γ = 1.67 ± 0.04. The 0.3-10 keV unabsorbed flux is estimated to be (6.2 ± 0.1) × 10-10 erg cm-2 s-1.
Observations with the Ultraviolet/Optical Telescope (UVOT) were carried out using the V filter with a total exposure of 2.6 ks. No counterpart to the X-ray source is detected in a highly crowded field. We derive a 3σ upper limit of V > 19.6 AB magnitude (flux density < 5.1 × 10-17 erg cm-2 s-1 A-1) in the stacked image.
We thank the Swift team for promptly scheduling and performing observations. Further multiwavelength observations are strongly encouraged to help constrain the nature of this new transient.
