Kavli Affiliate: Claudio Ricci
| First 5 Authors: Alessandro Peca, Michael J. Koss, Roberto Serafinelli, Claudio Ricci, C. Megan Urry
| Summary:
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.
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