A Radio, Optical, UV, and X-Ray View of the Enigmatic Changing-look Active Galactic Nucleus 1ES 1927+654 from Its Pre- to Postflare States

Laha, Sibasish; Meyer, Eileen; Roychowdhury, Agniva; Becerra Gonzalez, Josefa; Acosta-Pulido, J. A.; Thapa, Aditya; Ghosh, Ritesh; Behar, Ehud; Gallo, Luigi C.; Kriss, Gerard A.; Panessa, Francesca; Bianchi, Stefano; La Franca, Fabio; Scepi, Nicolas; Begelman, Mitchell C.; Longinotti, Anna Lia; Lusso, Elisabeta; Oates, Samantha; Nicholl, Matt; Cenko, S. Bradley
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The Astrophysical Journal

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The nearby Type II active galactic nucleus (AGN) 1ES 1927+654 went through a violent changing-look (CL) event beginning 2017 December 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 2018 July, 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 preflare state through mid-2021 with new and archival data from the Very Long Baseline Array, the European VLBI Network, the Very Large Array, the Telescopio Nazionale Galileo, Gran Telescopio Canarias, The Neil Gehrels Swift observatory, and XMM-Newton. The main results from our work are (i) the source has returned to its pre-CL state in optical, UV, and X-ray; the disk-corona relation has been reestablished as it has been in the pre-CL state, with an α OX ~ 1.02. The optical spectra are dominated by narrow emission lines. (ii) The UV light curve follows a shallower slope of ∝ t -0.91±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. (iii) The compact radio emission which we tracked in the pre-CL (2014), during CL (2018), and post-CL (2021) at spatial scales <1 pc was at its lowest level during the CL event in 2018, nearly contemporaneous with a low 2-10 keV emission. The radio to X-ray ratio of the compact source L Radio/L X-ray ~ 10-5.5 follows the Güdel-Benz relation, typically found in coronally active stars and several AGNs. (iv) We do not detect any presence of nascent jets at the spatial scales of ~5-10 pc.
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