Periodicities in the K2 lightcurve of HP Librae

TL;DR

This study analyzes Kepler/K2 data of HP Librae, detecting four key periodicities including the orbital, superhump, and disc precession frequencies, with implications for understanding helium accreting systems.

Contribution

First direct detection of the apsidal precession frequency in a helium accreting system's photometry, expanding knowledge of such systems' periodic behaviors.

Findings

Detected four periodicities including orbital, superhumps, and precession frequencies.

Measured positive superhump period change rate of 10^{-7} days/day.

Identified a stable 300 cyc/day QPO related to other QPOs.

Abstract

We analyse Kepler/K2 lightcurve data of the AM CVn system HP Librae. We detect with confidence four photometric periodicities in the system: the orbital frequency, both positive and negative superhumps, and the positive apsidal precession frequency of the accretion disc. This is only the second time that the apsidal precession frequency has ever been directly detected in the photometry of a helium accreting system, after SDSS J135154.46-064309.0. We present phase-folded lightcurves and sliding power spectra of each of the four periodicities. We measure rates of change of the positive superhump period of $10^{-7}$ days/day. We also redetect a QPO at 300 cyc/day, a feature that has been stable over decades, and show that it is harmonically related to two other QPOs, the lowest of which is centered on the superhump/orbital frequency. The continuum power spectrum is consistent with a single power law with no evidence of any breaks within our observed frequency range.