NuSTAR follow-up of the X-ray transient EP250916a
ATel #17421; G. K. Jaisawal, J. Chenevez (DTU Space), G. Illiano (INAF-OAB), A. Sanna (University of Cagliari), Q. C. Zhao (IHEP)
on 26 Sep 2025; 08:18 UT
Credential Certification: Gaurava Kumar Jaisawal (gaurava.jaisawal@gmail.com)
Subjects: X-ray, Transient
The X-ray transient EP250916a was discovered by the Einstein Probe/WXT on 2025 September 16 (ATel #17395). Follow-up observations with EP/FXT (GCN #41861), SVOM/MXT (ATel #17396), and Swift/XRT (ATel #17397) confirmed rapid brightening, with the spectrum consistently described by an absorbed power-law with photon index in the range 1.6-2.0 and hydrogen column density of (3-4) × 1021 cm-2.
We observed EP250916a with NuSTAR on 2025 September 24 between 11:25 and 22:15 UTC for a total exposure of 21 ks. The source was detected at R.A. = 268.239 deg, Dec. = -35.323 deg (J2000), consistent with the position reported by Swift/XRT (ATel #17397). The 3-79 keV light curve shows an average count rate of ~12 cts/s per focal plane module. No X-ray bursts or flares were observed during the observation. A preliminary search for pulsations and quasi-periodic oscillations in the 0.01-400 Hz range yielded no significant candidates.
The broadband 3-70 keV NuSTAR spectrum can be described by an absorbed power-law model. Preliminary best-fit parameters are: hydrogen column density nH (3 ± 2) × 1021 cm-2, photon index 2.03 ± 0.01, power-law normalization 0.15 ± 0.01, and reduced chi-squared of 1.15 for 1313 degrees of freedom. The unabsorbed fluxes are (4 ± 0.01) × 10-10 and (6.72 ± 0.01) × 10-10 erg cm-2 s-1 in the 0.3-10 and 3-70 keV ranges, respectively. Uncertainties on the spectral parameters are quoted at the 90% confidence level. Our spectral analysis does not reveal clear signatures of a soft X-ray thermal component, potentially associated with an accretion disk or boundary layer, nor the emission line in the 6-7 keV iron band.
The NuSTAR observation confirms that EP250916a remains in an active state and is mostly consistent with the properties of an accreting compact object in a hard state. Detailed analysis is ongoing. We encourage continued multiwavelength monitoring to constrain the nature of this transient source.
We thank the NuSTAR PI, Fiona Harrison, for approving the DDT request, and the NuSTAR SOC for promptly carrying out the observation.
