NuSTAR Detection of an Absorption Feature in ESP 39607: Evidence for an Ultra-Fast Inflow?
Alessandro Peca, Michael J. Koss, Roberto Serafinelli, Claudio Ricci, C. Megan Urry, Giulia Cerini, Peter G. Boorman
arXiv:2505.07963v2 Announce Type: replace
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.arXiv:2505.07963v2 Announce Type: replace
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.