2025/05/12 by Peca, Alessandro, Koss, Michael J., Serafinelli, Roberto +4 · 1 citation
#Astrophysics of Galaxies (astro-ph.GA) #FOS: Physical sciences #High Energy Astrophysical Phenomena (astro-ph.HE)
paper · doi:10.48550/arxiv.2505.07963
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 May 2023 and August 2024. The feature is detected in both observations with individual significance levels between 2 and 3σ, computed with multiple statistical methods. The combined probability of detecting it in both observations is \gtrsim4σ. The absorption feature is consistent with an ultra-fast 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 22-89 Rg, 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, were investigated but found unlikely. If confirmed, this detection represents a rare example of a UFI, providing valuable evidence into extreme and/or non-standard 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.