Multiwavelength monitoring and reverberation mapping of NGC 2617 at deepest minimum with a sharp upward turn during 2021–2024

• Published in: Monthly notices of the Royal Astronomical Society 2024-12, Vol.536 (3), p.2089-2102
• Main Authors: Oknyansky, V L, Brotherton, M S, Tsygankov, S S, Dodin, A V, Tatarnikov, A M, Du, P, Burlak, M A, Ikonnikova, N P, Metlov, V G, Belinski, A A, Shatsky, N I, Wang, J -M, Bao, D -W, Fang, F, Zhai, S, Fu, Y -X, Bai, H -R, Zastrocky, T E, Chelouche, D, Figaredo, C Sobrino, Kaspi, S, Gaskell, C M
• Format: Article
• Language: English
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stae2707

We present the results of a new X-ray to near-IR photometric and spectroscopic monitoring of the changing look active galactic nucleus NGC 2617 carried out from March 2022 to March 2024. We found significant variability at all wavelengths and in the intensities and profiles of the broad Balmer lines. Reverberation mapping was carried out for three observing seasons during 2021–2024. We obtained time delays of $\sim$4 d for the response of the H ${\beta }$ line to optical continuum variations. The X-ray variations correlate well with the UV and optical, with a few days lag for longer wavelengths. The K band lagged the B band by $\sim$ 15 d during the last three seasons, which is significantly shorter than the delays reported previously by the 2016 and 2017–2019 campaigns. Near-IR variability arises from two different emission regions: the outer part of the accretion disc and a more distant dust component. The $HK$-band variability is governed primarily by dust. The H ${\beta }$/H ${\alpha }$ ratio variations (for broad components only) correlate with the X-ray and UV fluxes. The spectral type changed from type 1.8 to type 1.9 during 2023 October–2024 January and then a more rapid change to type 1.5 occurred in 2024 February. We interpret these changes as a combination of two factors: changes in the accretion rate as a dominant cause but also the sublimation or recovery of dust along the line of sight.