The light curve simulations of the 2021 anomalous event in SS Cygni

TL;DR

This study uses numerical simulations to investigate the unusual 2021 light curve event in SS Cyg, finding that gas-stream overflow likely caused the anomaly and explaining high X-ray flux during quiescence.

Contribution

The paper demonstrates that gas-stream overflow is essential to reproduce the 2021 anomalous event in SS Cyg's light curve, providing new insights into dwarf nova behavior.

Findings

Gas-stream overflow reproduces the 2021 event in simulations.

Enhanced viscosity alone cannot fully explain the anomaly.

Gas-stream overflow may explain high X-ray flux during quiescence.

Abstract

The prototype dwarf nova SS Cyg unexpectedly exhibited an anomalous event in its light curve in the early few months of 2021 in which regular dwarf nova-type outbursts stopped, but small-amplitude fluctuations occurred only. Inspired by this event, we have performed numerical simulations of light curves of SS Cyg by varying mass transfer rates and varying viscosity parameters in the cool disk. We have also studied the effect of gas-stream overflows beyond the outer disk edge in the light curve simulations. We have confirmed that the enhanced mass transfer is unlikely responsible for the 2021 anomalous event and its forerunner. We have found that the enhancement of the viscosity in the disk may reproduce the forerunner of that event but may not be enough to explain the 2021 anomalous event, although the latter result might be particular to our thermal equilibrium curve used. Within our simulations, a model of the gas stream overflow with a slightly higher mass transfer rate than that of our standard model reproduces light curves similar to the 2021 anomalous event. We suggest that the gas-stream overflow is necessary to reproduce that event. The gas-stream overflow may also be responsible for the abnormally high X-ray flux during the normal quiescent state in SS Cyg.