NuSTAR Detection of an Absorption Feature in ESP 39607: Evidence for an Ultrafast Inflow?

We report the serendipitous discovery of an absorption feature at 4.8 keV in the NuSTAR spectra of ESP 39607, a Seyfert 2 galaxy at z = 0.201, observed in 2023 May and 2024 August. The feature is detected in both observations with individual significance levels between 2σ and 3σ, computed with multiple statistical methods. The combined probability of detecting it in both observations is ≳4σ. The absorption feature is consistent with an ultrafast inflow (UFI) potentially associated with Fe xxv or Fe xxvi Kα transitions. The inferred inflow velocity is ∼0.15–0.20c, with an estimated launching radius of 22–89R_g, depending on the assumed iron transition and whether radiation pressure is accounted for. Photoionization modeling associates the UFI primarily with Fe xxv Kα absorption, blended with a minor contribution from Fe xxvi Kα. Alternative explanations, including associations with the warm-hot intergalactic medium or outflows of lighter elements, are investigated but found unlikely. If confirmed, this detection represents a rare example of a UFI, providing valuable evidence into extreme and/or nonstandard accretion processes near supermassive black holes. Follow-up observations with higher-resolution X-ray spectroscopy, such as with XMM-Newton or XRISM, will be essential to confirm the nature of this feature and better constrain the physical mechanisms driving it.