GOTO065054+593624: a 8.5 mag amplitude dwarf nova identified in real time via Kilonova Seekers (2501.11524v2)

Abstract: Dwarf novae are astrophysical laboratories for probing the nature of accretion, binary mass transfer, and binary evolution -- yet their diverse observational characteristics continue to challenge our theoretical understanding. We here present the discovery of, and subsequent observing campaign on GOTO065054+593624 (hereafter GOTO0650), a dwarf nova of the WZ Sge type, discovered in real-time by citizen scientists via the Kilonova Seekers citizen science project, which has an outburst amplitude of 8.5 mag. An extensive dataset charts the photometric and spectroscopic evolution of this object, covering the 2024 superoutburst. GOTO0650 shows an absence of visible emission lines during the high state, strong H and barely-detected HeII emission, and high-amplitude echo outbursts with a rapidly decreasing timescale. The comprehensive dataset presented here marks GOTO0650 as a candidate period bouncer, and highlights the important contribution that citizen scientists can make to the study of Galactic transients.

• The paper analyzes the peculiar dwarf nova GOTO065054+593624, identified via citizen science, detailing its atypical photometric and spectroscopic behavior.
• Observations reveal unusual features including the absence of emission lines during the high state, peculiar superhump development, and a series of echo outbursts.
• These unique characteristics challenge existing WZ Sge-type dwarf nova models and highlight the significant role of citizen science in variable star astrophysics.

Analysis of GOTO065054+593624: A Peculiar Dwarf Nova Identified in Real Time Via Kilonova Seekers

The paper presents an in-depth analysis of GOTO065054+593624 (GOTO0650), a dwarf nova classified as a WZ Sge-type. This object was identified by citizen scientists through the Kilonova Seekers project, marking an instance of successful real-time collaboration between amateur astronomers and professional observatories. The extensive dataset collected over the first two months of its 2024 superoutburst provides new insights into its atypical photometric and spectroscopic behavior, challenging existing theoretical frameworks for cataclysmic variables (CVs).

Discovery and Initial Characteristics

GOTO0650 was detected by the Gravitational-wave Optical Transient Observer (GOTO) and quickly flagged as a significant transient event. The source exhibited a substantial optical brightness increase, with an outburst magnitude of approximately $m_V = 13.02$ and an amplitude of 8.5 magnitudes compared to its quiescent state. The precise timing measurements restrict the commencement of the outburst within a narrow time frame of 1.8 days. The photometric decline from its peak featured a gradual color change, hinting at complex disk dynamics.

Spectroscopic Features and Variability

High-resolution spectra from multiple facilities portrayed a blue continuum with prominent Balmer absorption, yet conspicuous by the absence of emission lines throughout the high state. This lack of emission is particularly unusual for WZ Sge-type novae, which typically exhibit strong line emissions reflective of active accretion processes. During the low state, GOTO0650 displayed narrow Balmer emission lines with broader absorption features indicative of an underlying accretion disk structure.

Superhump Phenomenon and Temporal Properties

Analysis of short-period variability unveiled the onset of superhumps approximately 14 days after the initial outburst, characterized by a period of $91 \pm 1$ minutes. Unusually, the superhump amplitude reached its peak post-dip before rapidly declining. The periodic behavior and amplitude evolution of these superhumps offer a powerful probe into the system's mass distribution and stability mechanisms.

Echo Outbursts and Elevated State

Following the main event, GOTO0650's light curve was marked by a sequence of eight echo outbursts. Each outburst consistently reached peak luminosity similar to the initial, but with a gradually shortening inter-recurrence interval—a behavior that underscores novel accretive processes not fully aligned with standard disk instability models. The elevated brightness baseline post-bounce suggests a non-trivial residual accretion or thermal instability process.

Implications for Cataclysmic Variable Models

The unique observational signatures of GOTO0650, particularly the presence of repeated echo outbursts combined with anomalous spectroscopic features, necessitate a reevaluation of WZ Sge models, especially concerning their post-main-outburst accretion dynamics. Furthermore, the lack of early superhumps observed challenges the inclination-based understanding of superhump evolution.

Conclusion and Prospective Research Directions

The observation of GOTO0650 underscores how citizen science initiatives can contribute significantly to variable star astrophysics. Future directions include phase-resolved spectroscopy to estimate binary parameters such as mass ratios and investigate possible period-bounce characteristics further. These could establish GOTO0650 as a prime candidate for studying evolutionary endpoints of short-period binaries, contributing to the broader understanding of low-mass transfer rates and evolutionary pathways in cataclysmic systems.