The Star That Blinks on Schedule express
The binary star KL Dra works like a neon clock: every two months it lights up for a few days, and between these outbursts it blinks with short but bright pulses. For eleven straight years, telescopes have kept their eyes on it, and now the whole mechanism is visible.
In this pair, one star is a dense white dwarf, and the other is an ordinary star that loses matter. The matter doesn't fall directly but first accumulates in a disk around the white dwarf, like water behind a dam. When enough accumulates, the disk becomes unstable and breaks through: that's the outburst. Large outbursts last on average 5.7 days. After them, the star "winks" several times: these are short rebrightenings. Then, a few weeks later, a series of three or four normal outbursts begins, each lasting just over a day.
The cycle repeats every 60.4 days. The longer the star "rests" after a large outburst, the shorter the subsequent series of rebrightenings, but the brighter the large outburst itself. Such precision allows us to test models: why the disk loses stability, how exactly matter falls onto the white dwarf, and why sometimes outbursts are skipped.
Original abstract
as written by the authors arXiv:2601.04323
We present the longest-term optical analysis of the AM CVn system KL Dra using $$\sim11$$ years of monitoring from TESS and wide-field ground-based surveys. The continuous TESS coverage allows us to characterise its frequent outbursts with unprecedented detail, providing the first comprehensive study of an AM CVn during outbursts and enabling detailed modelling of these systems. The superoutbursts in KL Dra generally include a precursor, and are followed by a series of rebrightenings after which a sequence of 3-4 large amplitude normal outbursts is observed. We fit parametric profiles to each superoutburst component (precursor, rise to plateau, plateau, decay), to rebrightenings, and to normal outbursts, which let us quantify every high state feature and investigate correlations with the system's long term supercyle evolution. Our continuous coverage reveals an average value for the supercycles, superoutbursts and normal outbursts of $$60.4 \pm 0.1$$ d, $$5.67\pm0.03$$ d and $$1.17 \pm0.01$$ d, respectively. The supercycle duration may be correlated with the rebrightenings duration and superoutburst amplitude, and anticorrelated with the plateau length. Within a supercycle, normal outbursts grow in amplitude and duration, and the first normal outburst is usually highly asymmetric, while subsequent normal outbursts are more symmetric. We detected superhumps in TESS superoutbursts but not in the rebrightenings or normal outbursts. We interpret the results within the disk instability model, considering additional effects, such as changes in the donor mass transfer rate.
The recurrence period of large outbursts in KL Dra is 60.4 days with an uncertainty of only 0.1 days. That's more precise than some train schedules.