NuSTAR Detection of an Absorption Feature in ESP 39607: Evidence for an Ultra-Fast Inflow?

TL;DR

This paper reports the detection of an absorption feature in the X-ray spectrum of a Seyfert galaxy, suggesting the presence of an ultra-fast inflow near the supermassive black hole, which is a rare and significant observation.

Contribution

It presents the first evidence of an ultra-fast inflow in ESP 39607 through X-ray spectroscopy, highlighting a potential new aspect of accretion physics.

Findings

Detection of a 4.8 keV absorption feature with >4σ significance

Inferred inflow velocity of approximately 0.15-0.20c

Association of the feature with Fe XXV Kα absorption

Abstract

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 3$\sigma$, computed with multiple statistical methods. The combined probability of detecting it in both observations is $\gtrsim$4$\sigma$. The absorption feature is consistent with an ultra-fast inflow (UFI) potentially associated with Fe XXV or Fe XXVI K$\alpha$ transitions. The inferred inflow velocity is $\sim$0.15-0.20$c$, with an estimated launching radius of 22-89 $R_g$, depending on the assumed iron transition and whether radiation pressure is accounted for. Photoionization modeling associates the UFI primarily with Fe XXV K$\alpha$ absorption, blended with a minor contribution from Fe XXVI K$\alpha$. 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.