{
  "id": "2505.07963",
  "version": "v1",
  "published": "2025-05-12T18:04:52.000Z",
  "updated": "2025-05-12T18:04:52.000Z",
  "title": "NuSTAR Detection of an Absorption Feature in ESP 39607: Evidence for an Ultra-fast Inflow?",
  "authors": [
    "Alessandro Peca",
    "Michael J. Koss",
    "Roberto Serafinelli",
    "Claudio Ricci",
    "C. Megan Urry",
    "Giulia Cerini",
    "Peter G. Boorman"
  ],
  "comment": "Submitted to ApJ. Comments are welcome",
  "categories": [
    "astro-ph.HE",
    "astro-ph.GA"
  ],
  "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.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2025-05-12T18:04:52.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "absorption feature",
      "ultra-fast inflow",
      "nustar detection",
      "fe xxvi",
      "observations"
    ],
    "note": {
      "typesetting": "TeX",
      "pages": 0,
      "language": "en",
      "license": "arXiv",
      "status": "editable"
    }
  }
}