A radio, optical, UV and X-ray view of the enigmatic changing look Active Galactic Nucleus 1ES~1927+654 from its pre- to post-flare states

Abstract: The nearby type-II AGN 1ES1927+654 went through a violent changing-look (CL) event beginning December 2017 during which the optical and UV fluxes increased by four magnitudes over a few months, and broad emission lines newly appeared in the optical/UV. By July 2018 the X-ray coronal emission had completely vanished, only to reappear a few months later. In this work we report the evolution of the radio, optical, UV and X-rays from the pre-flare state through mid-2021 with new and archival data from the Very Long Baseline Array (VLBA), the European VLBI Network, the Very Large Array (VLA), the Telescopio Nazionale Galileo (TNG), Gran Telescopio Canarias (GTC), The Neil Gehrels Swift observatory and XMM-Newton. The main results from our work are: (1) The source has returned to its pre-CL state in optical, UV, and X-ray; the disk-corona relation has been re-established as has been in the pre-CL state, with an $\alpha_{\rm OX}\sim 1.02$. The optical spectra are dominated by narrow emission lines. (2) The UV light curve follows a shallower slope of $\propto t^{-0.91\pm 0.04}$ compared to that predicted by a tidal disruption event. We conjecture that a magnetic flux inversion event is the possible cause for this enigmatic event. (3) The compact radio emission which we tracked in the pre-CL (2014), during CL (2018) and post-CL(2021) at spatial scales $&lt;1$ pc was at its lowest level during the changing look event in 2018, nearly contemporaneous with a low $2-10$ keV emission. The radio to X-ray ratio of the compact source $L_{\rm Radio}/L_{\rm X-ray}\sim 10<sup>{-5.5}$, follows the Gudel-Benz relation, typically found in coronally active stars, and several AGN. (4) We do not detect any presence of nascent jets at the spatial scales of $\sim 5-10$ pc.

• The paper demonstrates that the AGN's disk-corona system robustly recovers post-flare, evidenced by restored optical, UV, and X-ray emission characteristics.
• The analysis of the UV light curve reveals a decay slope of approximately t^(-0.91), challenging classical tidal disruption event models.
• Multi-wavelength observations link the disappearing radio emission and evolving optical lines to potential magnetic flux inversion and accretion dynamics.

Overview of the Changing Look AGN 1ES 1927+654

This paper investigates the enigmatic changing-look AGN 1ES 1927+654, documenting its evolution from pre-flare to post-flare states across multiple wavelengths including radio, optical, UV, and X-rays. This AGN, located at a redshift of 0.017, is notable for its dramatic variability, having undergone a violent changing-look event beginning in December 2017. Over a short time, the optical and UV emissions increased by four magnitudes, with new broad emission lines appearing. This study utilizes data from various observatories such as the Very Long Baseline Array, European VLBI Network, Very Large Array, Telescopio Nazionale Galileo, Gran Telescopio Canarias, and the Neil Gehrels Swift Observatory.

Key Findings

1. Return to Pre-CL State: The AGN's optical, UV, and X-ray emissions returned to their pre-change state, re-establishing the disk-corona relationship characterized by an $\alpha_{\rm OX} \sim 1.02$. This suggests the disk-corona mechanics are robust and return to equilibrium after disturbance.
2. UV Light Curve Analysis: The UV light curve follows a decay with a slope of $\propto t^{-0.91 \pm 0.04}$, which differs from the classical $t^{-5/3}$ predicted by tidal disruption events (TDE). This behavior raises questions about the exact nature of the event affecting this AGN.
3. Theoretical Implications: The peculiar behavior observed may be linked to a magnetic flux inversion event. The separate evolutions of optical/UV and X-ray emissions imply control by distinct physical parameters potentially related to mass accretion rate and magnetic flux on the black hole.
4. Radio Emission Insights: During the changing look phase, the compact radio emission was at its lowest, aligning with a simultaneous dip in X-ray emission. This supports the hypothesis connecting radio emissions to the X-ray corona, as the radio to X-ray ratio follows the Gudel-Benz relation common in coronally active stars.
5. Analysis of Optical Spectra: Between pre-flare and post-flare states, the optical spectra showed variability in emission lines. Notably, a weak broad H$\alpha$ line in 2011 suggests a persistent, albeit faint, BLR, contesting the notion of 1ES 1927+654 as a true type-II AGN without an obscured broad-line region.
6. Implications for AGN Models: The absence of correlation between soft and hard X-ray fluxes and the lack of changes typical of TDEs suggests a more complex interplay possibly involving magnetic fields. The revival of the X-ray corona and persistent soft X-ray excess also challenge existing reflection-based models for the latter's origin.

Future Outlook

This study opens avenues for further exploration into the nature of AGN variability, particularly the role of magnetic fields in changing-look phenomena. High-cadence observations, especially in the radio and X-ray bands, could refine our understanding of corona dynamics and the feed-back mechanisms in AGN. Additionally, further investigations into nascent jet formations and wind-driven mechanisms could elucidate the extended radio emissions detected post-flare. By addressing these questions, future research could considerably enhance our comprehension of AGN accretion systems and their inherent complexities.