A New Stellar Outburst Associated with the Magnetic Activities of the K-type Dwarf in a White-dwarf Binary

TL;DR

This paper investigates a unique stellar outburst in a binary system, linking magnetic activity of the secondary star to sudden mass transfer events, challenging traditional views on cataclysmic variable outbursts.

Contribution

It provides evidence that magnetic activity on the secondary star can quench mass transfer, leading to outbursts driven by sudden accretion, offering new insights into CV behavior.

Findings

Magnetic spots on the secondary star correlate with outburst events.

The white dwarf was not accreting material prior to the outburst.

Outburst caused by sudden mass accretion after magnetic activity quenching.

Abstract

1SWASP\,J162117.36$+$441254.2 was originally classified as an EW-type binary with a period of 0.20785\,days. However, it was detected to have undergone a stellar outburst on June 3, 2016. Although the system was latter classified as a cataclysmic variable (CV) and the event was attributed as a dwarf-nova outburst, the physical reason is still unknown. This binary has been monitored photometrically since April 19, 2016 and many light curves were obtained before, during and after the outburst. Those light and color curves observed before the outburst indicate that the system is a special CV. The white dwarf is not accreting material from the secondary and there are no accretion disks surrounding the white dwarf. By comparing the light curves obtained from April 19 to September 14, 2016, it was found that magnetic activity of the secondary is associated with the outburst. We show strong evidence that the $L_1$ region on the secondary was heavily spotted before and after the outburst and thus quench the mass transfer, while the outburst is produced by a sudden mass accretion of the white dwarf. These results suggest that J162117 is a good astrophysical laboratory to study stellar magnetic activity and its influences on CV mass transfer and mass accretion.