The Intermediate Polar FO Aquarii Has Not Been the Same Since Recovering from a Series of Low States

TL;DR

FO Aqr, an intermediate polar, has shown significant changes in its photometric behavior after recovering from low states, indicating a possible new normal in its spin and beat pulse characteristics.

Contribution

This study compares historical and recent photometry of FO Aqr, revealing changes in its spin and beat pulse features post-recovery from low states.

Findings

Weakening of the spin pulse in 2021 data

Enhanced beat pulse in recent observations

Presence of variations at twice the spin frequency

Abstract

FO Aqr is a bright intermediate polar that has long displayed large amplitude photometric variations corresponding to the 20.9 min spin period of its white dwarf. Between 2016 and 2020, the system suffered a series of unprecedented low-states, but recent data shows that it has now recovered to its normal optical luminosity. We compare the light curves obtained by K2/Kepler in 2014 with photometry from the TESS mission obtained in 2021. We find that the spin pulse that had been the dominant feature of the light curve in 2014 is now weak over the second half the binary orbit and that a beat pulse is enhanced in the TESS photometry. Variations at approximately twice the spin frequency are now seen over the second half of the orbit. These photometric properties may be the new normal for FO Aqr now that its white dwarf has begun to spin down.