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

中文翻译：

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2021年矮新星天鹅座SS异常事件的性质及其多波长跃迁

SS Cyg 长期以来一直被认为是一组只显示爆发的矮新星的原型。然而，这个天体在 2021 年进入了一个相当反常的事件，起初似乎是静止的，即在 Z Cam 型矮新星中观察到的几乎恒定的光度状态。这一意外事件为我们提供了一个很好的机会来重新考虑灾难性变量中停顿的性质。我们通过许多光学望远镜和 X 射线望远镜 NICER 和 NuSTAR 观察到了这一异常事件及其前兆，即静止期间光学和 X 射线通量的逐渐和同时增加。在异常事件之前的静止期间，我们没有发现任何轨道驼峰的放大，这表明传质速率平均没有显着波动。估计的 X 射线通量不足以解释在静止期间通过 X 射线辐照圆盘和次星恒星的光通量增加。很自然地认为，在异常事件发生之前，静止盘中的粘度增加了，这增加了盘中的质量吸积率，不仅提高了光通量，还提高了 X 射线通量。我们认为，粘度的增加也引发了 SS Cyg 中的类似静止现象，这被认为是一系列小爆发。圆盘的内部将始终处于爆发状态，并且只有其外部在此现象期间会因热粘性不稳定性而不稳定，这与观察到的光学颜色变化是一致的。