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SS Cyg has long been recognized as the prototype of a group of dwarf novaethat show only outbursts. However, this object has entered a quite anomalousevent in 2021, which at first appeared to be standstill, i.e., an almostconstant luminosity state, observed in Z Cam-type dwarf novae. This unexpectedevent gives us a great opportunity to reconsider the nature of standstill incataclysmic variables. We have observed this anomalous event and itsforerunner, a gradual and simultaneous increase in the optical and X-ray fluxduring quiescence, through many optical telescopes and the X-ray telescopesNICER and NuSTAR. We have not found any amplification of the orbital humpduring quiescence before the anomalous event, which suggests that the masstransfer rate did not significantly fluctuate on average. The estimated X-rayflux was not enough to explain the increment of the optical flux duringquiescence via X-ray irradiation of the disk and the secondary star. It wouldbe natural to consider that viscosity in the quiescent disk was enhanced beforethe anomalous event, which increased mass accretion rates in the disk andraised not only the optical flux but also the X-ray flux. We suggest thatenhanced viscosity also triggered the standstill-like phenomenon in SS Cyg,which is considered to be a series of small outbursts. The inner part of thedisk would always stay in the outburst state and only its outer part would beunstable against the thermal-viscous instability during this phenomenon, whichis consistent with the observed optical color variations. This scenario is inline with our X-ray spectral analyses which imply that the X-ray emitting inneraccretion flow became hotter than usual and vertically expanded and that itbecame denser and was cooled down after the onset of the standstill-like state.

On the nature of the anomalous event in 2021 in the dwarf nova SS Cygni and its multi-wavelength transition