PM J03338+3320: Long-Period Superhumps in Growing Phase Following a Separate Precursor Outburst

TL;DR

This study documents the first observation of long-period superhumps during the growing phase after a precursor outburst in a cataclysmic variable, supporting the thermal-tidal instability model.

Contribution

It provides the first clear evidence that the 3:1 resonance is triggered by a precursor outburst, confirming key predictions of the thermal-tidal instability model.

Findings

Long-period superhumps observed between precursor and main superoutburst.

Superhump period is 6.0% longer than orbital period.

Supports the thermal-tidal instability model over alternative models.

Abstract

We observed the first-ever recorded outburst of PM J03338+3320, the cataclysmic variable selected by proper-motion survey. The outburst was composed of a precursor and the main superoutburst. The precursor outburst occurred at least 5 d before the maximum of the main superoutburst. Despite this separation, long-period superhumps were continuously seen between the precursor and main superoutburst. The period of these superhumps is longer than the orbital period by 6.0(1)% and can be interpreted to reflect the dynamical precession rate at the 3:1 resonance for a mass ratio of 0.172(4). These superhumps smoothly evolved into those in the main superoutburst. These observations provide the clearest evidence that the 3:1 resonance is triggered by the precursor outburst, even if it is well separated, and the resonance eventually causes the main superoutburst as predicted by the thermal-tidal instability model. The presence of superhumps well before the superoutburst cannot be explained by alternative models (the enhanced mass-transfer model and the pure thermal instability model) and the present observations give a clear support to the thermal-tidal instability model.