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

Abstract

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.

Y Gem: A White Dwarf Symbiotic Star?

“…For a disk-accreting WD, the mass of the WD (M WD ) determines its gravitational potential, and therefore the maximum shock temperature (T max ). The T max is commonly obtained from the spectral fitting of the X-ray continuum (e.g., Kimura et al 2021), but it is difficult to constrain the WD mass without 10 keV X-ray spectra. Fortunately, the iron line flux ratio could be an indicator on T max , and therefore the WD mass.…”

A Broadband X-Ray Study of the Dwarf Nova SS Cyg during Quiescence and Outburst

“…A recent study (Kimura et al 2021) of an anomalous outburst event of SS Cyg in 2021 using simultaneous NICER and NuSTAR data showed a weak reflection (∼0.2) amplitude during the anomalous outburst event as well as during the quiescence phase before it. They proposed a large shock height where the hard X-ray-emitting corona is greatly expanded in the vertical direction, similar to a coronal siphon flow scenario, to an extent that is more than the WD radius, resulting in a weak reflection (their Figure 10).…”

Searching for Anomalies in the ZTF Catalog of Periodic Variable Stars

“…These may potentially contain information about new physics. Indeed, the discoveries of anomalous periodic variables have been challenging our understanding of the chemical composition of stars (Niemczura et al 2017;Bernhard et al 2021), the stellar evolution models (Groenewegen & Jurkovic 2017), Galactic metallicity (Jurkovic 2018), the physics of accretion and mass transfer (Gautschy & Saio 2017;Kimura et al 2021), the physics of active galactic nuclei (AGNs; Sánchez-Sáez et al 2021), and even the formation of low-mass white dwarfs (Masuda et al 2019).…”