Outbursts in ultra-compact AM CVn binaries

TL;DR

The paper addresses how outbursts in ultra-compact AM CVn binaries depend on orbital period and disc physics. It leverages the Green et al. (2025) catalogue and KL Dra as a detailed testbed, combining multiwavelength observations (including TESS and XMM-Newton) with disc instability model (DIM) predictions and mass transfer instability model (MTIM) expectations. A key finding is that recurrence times of outbursts broadly follow DIM predictions, though the inferred disc mass at quiescence end is somewhat lower than earlier assumptions; durations and amplitudes are highly sensitive to observational cadence and filter, complicating direct comparisons and calling for standardized benchmarks. The study highlights the AM CVn class as a critical laboratory for discriminating accretion disc physics in hydrogen-deficient systems and motivates further systematic modelling and uniform light-curve analyses across a wide period range.

Abstract

AM CVn binaries are the most compact of accreting binaries having orbital periods in the range ~5-70 min. They consist of a white dwarf accreting hydrogen deficient material from a degenerate or semi-degenerate star and are predicted to be amongst the verification sources for future gravitational wave observatories such as LISA. Using the recent catalogue of Green et al (2025) I focus attention on the orbital period range in which outbursts are seen from AM CVn's. I examine in more detail the outburst properties of KL Dra which has an outburst every few months and has many sectors of TESS data as an open resource. Using observational data on the outbursting systems in general, I compare the outburst recurrence time, duration and amplitude as a function of orbital period with the predictions of the disc instability model. The recurrence time is well described, although there is some evidence that the amount of material in the disc at the end of the quiescence phase is less than earlier model assumptions. The distribution of the outburst duration appears to be dependent on the cadence of the observations and how it is defined. Similarly the amplitude distribution is dependent on cadence and the filter, which causes an apparent spread in distribution. Both of these features need to be systematically studied using consistent benchmarks. AM CVn binaries remain an excellent sources to test models which aim to predict the properties of disc accreting systems.

Figures (9)

• Figure 1: The number of confirmed AM CVn binaries as a function of year where the size of the symbol reflects the stars orbital or superhump period (data taken from Green2025) For systems with no confirmed or predicted period we have set their symbol size to zero.
• Figure 2: The confirmed systems in the catalogue of Green2025 split into direct imapact, high state, outbursters and low state systems.
• Figure 3: Photometry of KL Dra obtained using the Liverpool Telescope between Aug 2009 and July 2011 (data from Ramsay2012a) showing outbursts every few months.
• Figure 4: TESS data of KL Dra obtained in sector 55 showing one normal outburst and one superoutburst (which immediately follows a second normal outburst) obtained from the TESS SPOC pipeline (Caldwell2020).
• Figure 5: Observations of KL Dra made using the Liverpool Telescope (optical); XMM-Newton OM (UV) and XMM-Newton EPIC (X-rays) (Adapted from Ramsay2012b).