Return of the King: Time-Series Photometry of FO Aquarii's Initial Recovery from its Unprecedented 2016 Low State

TL;DR

This study presents high-speed photometry of FO Aqr during its recovery from a rare low state, revealing changes in accretion modes, slow recovery rate, and brightness-dependent pulse timings, providing insights into its accretion dynamics.

Contribution

First detailed time-resolved photometry during FO Aqr's low state recovery, showing a shift from disk-fed to stream-fed accretion and characterizing its slow, irregular recovery process.

Findings

Detection of strong periodicities at 22.5 min and 11.26 min.

Evidence of stream-fed accretion during recovery.

Unusually slow recovery with an e-folding time of 115 days.

Abstract

In 2016 May, the intermediate polar FO~Aqr was detected in a low state for the first time in its observational history. We report time-resolved photometry of the system during its initial recovery from this faint state. Our data, which includes high-speed photometry with cadences of just 2 sec, shows the existence of very strong periodicities at 22.5 min and 11.26 min, equivalent to the spin-orbit beat frequency and twice its value, respectively. A pulse at the spin frequency is also present but at a much lower amplitude than is normally observed in the bright state. By comparing our power spectra with theoretical models, we infer that a substantial amount of accretion was stream-fed during our observations, in contrast to the disk-fed accretion that dominates the bright state. In addition, we find that FO~Aqr's rate of recovery has been unusually slow in comparison to rates of recovery seen in other magnetic cataclysmic variables, with an $e$-folding time of 115$\pm7$ days. The recovery also shows irregular variations in the median brightness of as much as 0.2~mag over a 10-day span. Finally, we show that the arrival times of the spin pulses are dependent upon the system's overall brightness.