Wang, YananYananWangPasham, Dheeraj R.Dheeraj R.PashamAltamirano, DiegoDiegoAltamiranoGúrpide, AndrésAndrésGúrpideCastro Segura, NoelNoelCastro SeguraMiddleton, MatthewMatthewMiddletonJi, LongLongJidel Palacio, SantiagoSantiagodel PalacioGuolo, MuryelMuryelGuoloGandhi, PoshakPoshakGandhiZhang, Shuang-NanShuang-NanZhangRemillard, RonaldRonaldRemillardLin, DachengDachengLinMasterson, MeganMeganMastersonBALDI, Ranieri DiegoRanieri DiegoBALDITOMBESI, FrancescoFrancescoTOMBESIMiller, Jon M.Jon M.MillerZhang, WendaWendaZhangSANNA, ANDREAANDREASANNA2024-12-302024-12-3020240004-637Xhttp://hdl.handle.net/20.500.12386/35588The tidal disruption of a star around a supermassive black hole (SMBH) offers a unique opportunity to study accretion onto an SMBH on a human timescale. We present results from our 1000+ days monitoring campaign of AT 2019avd, a nuclear transient with tidal-disruption-event-like properties, with NICER, Swift, and Chandra. Our primary finding is that approximately 225 days following the peak of the X-ray emission, there is a rapid drop in luminosity exceeding 2 orders of magnitude. This X-ray dropoff is accompanied by X-ray spectral hardening, followed by a plateau phase of 740 days. During this phase, the spectral index decreases from 6.2 ± 1.1 to 2.3 ± 0.4, while the disk temperature remains constant. Additionally, we detect pronounced X-ray variability, with an average fractional rms amplitude of 47%, manifesting over timescales of a few dozen minutes. We propose that this phenomenon may be attributed to intervening clumpy outflows. The overall properties of AT 2019avd suggest that the accretion disk evolves from a super-Eddington to a sub-Eddington luminosity state, possibly associated with a compact jet. This evolution follows a pattern in the hardness–intensity diagram similar to that observed in stellar-mass BHs, supporting the mass invariance of accretion–ejection processes around BHs.STAMPAenArticle10.3847/1538-4357/ad182b2-s2.0-85184907137https://iopscience.iop.org/article/10.3847/1538-4357/ad182bhttps://api.elsevier.com/content/abstract/scopus_id/851849071372024ApJ...962...78WFIS/05 - ASTRONOMIA E ASTROFISICA