MASTER OT J030227.28+191754.5: An unprecedentedly energetic dwarf nova outburst

Abstract

We present a detailed study of the MASTER OT J030227.28$+$191754.5 outburst in 2021–2022, which reached an amplitude of $10.2\:$mag and a duration of $60\:$d. The detections of (1) the double-peaked optical emission lines, and (2) the early and ordinary superhumps, established that MASTER OT J030227.28$+$191754.5 is an extremely energetic WZ Sge-type dwarf nova (DN). Based on the superhump observations, we obtained its orbital period and mass ratio as $0.05986(1)\:$d and 0.063(1), respectively. These values are within a typical range for low-mass-ratio DNe. According to the binary parameters derived based on the thermal–tidal instability model, our analyses showed that (1) the standard disk model requires an accretion rate $\simeq\!\! 10^{20}\:$g$\:$s$^{-1}$ to explain its peak optical luminosity, and (2) large mass was stored in the disk at the outburst onset. These factors cannot be explained solely by the impact of its massive ($\gtrsim\!\! 1.15\, M_{\odot }$) primary white dwarf implied by Kimura et al. (2023, ApJ, 951, 124). Instead, we propose that the probable origin of this enormously energetic DN outburst is the even lower quiescence viscosity than other WZ Sge-type DNe. This discussion is qualitatively valid for most possible binary parameter spaces unless the inclination is low enough ($\lesssim\!\! 40^\circ$) for the disk to be bright, explaining the outburst amplitude. Such low inclinations, however, would not allow detectable amplitude of early superhumps in the current thermal–tidal instability model. The optical spectra at outburst maximum showed strong emission lines of the Balmer, He i, and He ii series, the core of which is narrower than $\sim \! 800\:$km$\:$s$^{-1}$. Considering its binary parameters, a Keplerian disk cannot explain this narrow component, but the presumable origin is disk winds.