The outburst of a 60 min AM CVn reveals peculiar color evolution: implications for outbursts in long period double white dwarfs

Abstract: We report on multi-wavelength observations during quiescence and of the first detected outburst of the ~60 min orbital period AM CVn SDSS J113732+405458. Using X-ray and UV observations we determined an upper limit duration of the event of about one year. The amplitude of the outburst was remarkably small, of around one magnitude in r and 0.5 magnitudes in g. We have also investigated the color variations of SDSS J113732+405458 and other long period AM CVns in outbursts and identified a track on the color-magnitude diagram that is not compatible with the predictions of the disk instability model, suggesting that some outbursts in long period AM CVns are caused by enhanced mass-transfer. To our knowledge, these are the first studies of the color evolution in AM CVns. During quiescence we measured an X-ray luminosity for SDSS J113732+405458 of ~3E29 erg/s in the 0.5-10 keV band. This indicates a very low accretion rate, in agreement with the disk instability model for long period systems. However, such a model predicts stable disks at somewhat long periods. The discovery of this system outburst, along with similarities to the long period system SDSS J080710+485259 with a comparably long, weak outburst, indicates that these enhanced mass-transfer events may be more common in long period AM CVns. A larger sample would be needed to determine empirically at what period, if any, the disk instability stops functioning entirely. Finally, we identified an infrared excess in the quiescence spectrum attributable to the donor. This makes SDSS J113732+405458 the second AM CVn to have a directly detected donor.

• The paper reveals that the peculiar color evolution during the outburst of SDSS 1137 points to enhanced mass transfer, diverging from standard disk instability models.
• The study employs multi-wavelength data from X-rays to optical bands, showing a minimal amplitude outburst lasting longer than predicted for long-period systems.
• The results imply a need to revise accretion models and explore alternative mechanisms such as donor irradiation and circumbinary disk influences.

An Analysis of the Outburst in AM CVn System SDSS~J113732+405458

Overview

The research paper provides an in-depth study of the first detected outburst of the AM CVn system SDSS~J113732+405458 (SDSS 1137), a binary with an orbital period of approximately 60 minutes. This study utilized multi-wavelength observations to analyze both quiescent states and the active outburst of this helium-rich binary system, with a primary focus on understanding the mechanisms driving such outbursts in long-period AM CVn binaries. The findings indicate peculiar color evolution during the outburst, challenging traditional models such as the Disk Instability Model (DIM) and suggesting alternative theories involving enhanced mass-transfer from the donor.

Observational Insights

The researchers conducted observations using a variety of instruments spanning from X-rays (via XMM-Newton and Swift), ultraviolet, to optical bands (through Zwicky Transient Facility, ZTF). During the quiescent phase, X-ray luminosity was notably low, leading to a conclusion of a low accretion rate consistent with expectations from the DIM. The outburst was recorded as having a minimal amplitude—around one magnitude in the optical $r$-band—lasting for less than a year, which is conspicuously long compared to predictions from the DIM for long-period systems.

Peculiar Color Variations

The paper's analysis highlighted significant deviations in SDSS~1137's color-magnitude path during the outburst when compared against the DIM predictions. Unlike shorter period AM CVn systems, whose outbursts are understood to arise from disk instabilities, the peculiar color change exhibited by SDSS~1137 suggests an outburst mechanism driven by enhanced mass-transfer from the donor star. This distinguishes it from expected behavior based on established theoretical models, especially for a system with such a long orbital period.

Implications and Theoretical Considerations

The implications of this study necessitate a re-evaluation of how long-period AM CVn systems transition into outburst phases. With enhanced mass transfer assumed to be a primary driver, mechanisms such as irradiation of the donor star or magnetically-driven cycles are potential avenues for further exploration. The apparent failure of the DIM in explaining the phenomena observed in SDSS~1137, and similarly in SDSS~J080710+485259, prompts consideration of whether the existing models can be modified or if new models are required to comprehend long-period systems better.

Future Directions

Further empirical investigations into long-period AM CVn systems are crucial. Specifically, obtaining a larger sample size of such systems undergoing outbursts will aid in determining whether there exists an empirical threshold where the usual disk instability ceases to govern the behavior of these binaries. Additionally, the authors suggest exploring the possible existence and impact of circumbinary disks that might contribute to the observed behavior in infrared bands.

Conclusion

This paper provides significant contributions to the field by expanding our understanding of long-period AM CVn systems. The discovery and analysis of the outburst in SDSS 1137 challenge conventional models and open up various speculative but promising areas for further research. Identifying an isolated NIR excess attributable to the donor further enriches the dialogue on direct detection techniques and their potential utility in characterizing binary components more precisely. As more data become available, the insights drawn here will inevitably shape the dialogue surrounding accretion dynamics and the lifecycle of AM CVn stars.