MASTER OT J030227.28+191754.5: an unprecedentedly energetic dwarf nova outburst (2408.13783v1)

Abstract: We present a detailed study of the MASTER OT J030227.28+191754.5 outburst in 2021-2022, reaching 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 are within a typical range of 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 cannot be explained solely by the impact of its massive ($\gtrsim$ 1.15 M$_\odot$) primary white dwarf implied by Kimura et al. (2023). 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 ($\lesssim 40^\circ$) enough 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 the strong emission lines of Balmer, He I, and He II series whose core 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.

• The paper reports an unprecedented superoutburst with a 10.2 magnitude increase over 60 days in MASTER OT J030227.28+191754.5.
• It details observations of double-peaked emission lines and superhumps, supporting a unique accretion and disk wind phenomenon.
• The study constrains binary parameters with an orbital period of 0.05986 days and a mass ratio of 0.063, refining models of dwarf novae.

An Overview of MASTER OT J030227.28+191754.5 Superoutburst

This essay analyzes the observational paper conducted on the unprecedentedly energetic dwarf nova outburst of MASTER OT J030227.28+191754.5, as reported in the provided paper. The primary aim of the paper is to evaluate the characteristics and energetic implications of this outburst within the context of WZ Sge-type dwarf novae.

Key Observational Findings

1. Outburst Properties: The MASTER OT J030227.28+191754.5 experienced a superoutburst in 2021-2022, marked by an extraordinary amplitude of 10.2 magnitudes and a prolonged duration of 60 days. This is significantly larger and more sustained than typical WZ Sge-type dwarf novae.
2. Spectral Characteristics: Observations captured double-peaked emission lines and the presence of early and ordinary superhumps, indicative of extreme accretion activity. The noted emission lines include Balmer series, He I, and He II, with core widths narrower than approximately 800 km/s, suggesting the presence of disk winds rather than a Keplerian disk contribution.
3. Binary System Parameters: The orbital period and mass ratio were identified as 0.05986 days and 0.063, respectively. These values are consistent with low-mass-ratio systems typically found in WZ Sge-type dwarf novae.

Theoretical Implications and Discussions

1. Accretion Dynamics and Disk Instabilities: The paper posits that the outburst’s energetic nature cannot be solely attributed to the massive primary white dwarf (≥ 1.15 M☉). Instead, an exceptionally low quiescence viscosity within the disk is posited as a driving factor for the outburst characteristics. This enhances both the peak luminosity and stored mass in the accretion disk, aligning with the thermal-tidal instability model.
2. Disk Winds Hypothesis: The presence of narrow emission lines is suggested to be indicative of disk winds. This adds a layer of complexity to our understanding of accretion dynamics in dwarf novae, proposing an acceleration mechanism that contributes to mass ejection and could further explain the high-energy phenomena observed.
3. Comparative Analysis with Other Systems: In comparing MASTER OT J030227.28+191754.5 with other WZ Sge-type systems, the research provides insight into the extremities of dwarf novae behavior under specific mass and viscosity configurations. This comparison outlines the possible evolutionary pathways and highlights the role of intrinsic system parameters in shaping outburst profiles.

Speculation on Future Developments

This research opens up pathways for future observational and theoretical work to further dissect dwarf novae outbursts. The identification of disk wind signatures could prompt more focused studies on the interactions between disk winds and other accretion phenomena. Additionally, understanding viscosity's role in such systems could refine models predicting accretion behavior, offering broader implications across various astrophysical disk systems.

In conclusion, the paper effectively underscores the complexity and diversity within WZ Sge-type dwarf novae. Its implications reach beyond just a singular event, offering a deeper comprehension of disk dynamics essential for broader accretion physics. The results call for further exploration into the low-viscosity regime's impact on accretion and outburst mechanics, potentially leading to refinements in theoretical models of binary evolution and accretion-driven phenomena in varied stellar environments.