The Emergence of X-Ray Emission Lines during Relativistic Radio-jet Formation in the Changing-look Active Galactic Nucleus 1ES 1927+654

We present results from a comprehensive multiwavelength monitoring campaign of the changing-look active galactic nucleus 1ES 1927+654 during the onset and evolution of a relativistic radio jet ∼(2022 May–2025 August), using observations from XMM-Newton, Swift, Telescopio Nazionale Galileo (TNG), the Zwicky Transient Facility (ZTF), Very Large Array (VLA), and Very Long Baseline Array (VLBA). The soft X-ray emission lines at ∼0.56 keV and ∼1 keV have appeared with variable strength and width during the formation of the nascent jet. We note that the ∼1 keV feature has been persisting since the post-2017 flare phase. We also report the detection of a broad (∼800 eV) FeK emission feature at (6–7) keV in the ∼70 ks stacked EPIC-pn spectra, marking the first such detection, which historically was lacking in this source. The joint spectral fitting of XMM-Newton EPIC-pn and Reflection Grating Spectrometer (RGS) data reveals the presence of ionized absorbers in 2022 ( log ξ / erg cm s − 1 ∼ 1.5 ± 0.3 , N H ∼ 2.5 ± 0.9 × 10 20 cm −2 ), but weaker than that detected during the high accretion state in 2018 (Eddington ratio, λ Edd > 1). The absorption features further weakened in 2023–2025 and were marginally detectable ( N H ≤ 10 20 cm −2 ). The entire scenario is suggestive of a real-time transition of the accretion flow (from λ Edd > 1 to λ Edd ∼ 0.3) during which the winds become weaker and the jet starts to form and evolve. Furthermore, both the soft X-ray (0.3–2) keV and 5 GHz radio fluxes, which increased by factors of ∼10 and ∼60, respectively, since 2022, have recently plateaued at elevated levels, indicating a stabilized accretion disk, corona, and jet configuration.