Return of the King: Time-Series Photometry of Fo Aquarii’s Initial Recovery From Its Unprecedented 2016 Low State

Abstract

Publication date 2016-12-09Original citation Colin, L. et al (2016) 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 s, show the existence of very strong periodicities at 22.5 and 11.26 minutes, 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 streamfed 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±7 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.

The disappearance and reformation of the accretion disc during a low state of FO Aquarii

“…In the case of discless accretion, the power spectrum should be dominated by the beat frequency ω − Ω between the spin and orbital frequency and its first harmonic 2ω − 2Ω, whereas in the case of disc accretion, the spin frequency should be dominant (Ferrario & Wickramasinghe 1999). Littlefield et al (2016) found that, in the low state, the beat component amplitude is more than twice that of the spin component, while in the hight state, it is only 20% of the spin component. Although the modelling of the power spectrum is complex due to the complex geometry of the accretion flow and to the contribution of several sources to the optical light (see above), the change in the power spectrum clearly supports our finding that the disc disappears during the low state.…”

“…It is interesting to note that the partial eclipse shape changed during the low state; the eclipses became narrower and less deep, indicating that the accretion disc had shrunk (Littlefield et al 2016). As the phased light curve over the orbital period was calculated by Littlefield et al (2016) over their entire dataset, it is not possible to decide from these observations if the accretion disc completely disappeared at some point.…”

“…It is interesting to note that the partial eclipse shape changed during the low state; the eclipses became narrower and less deep, indicating that the accretion disc had shrunk (Littlefield et al 2016). As the phased light curve over the orbital period was calculated by Littlefield et al (2016) over their entire dataset, it is not possible to decide from these observations if the accretion disc completely disappeared at some point. One nevertheless expects, however, that material impacting the magnetosphere should form some structure in the orbital plane, possibly mimicking a ring whose size should be comparable to the magnetospheric radius, which is smaller by a factor ∼2 than the disc size in the normal state; this is approximately the ratio of the eclipse full width at half maximum (FWHM) in the normaland low states.…”

“…FO Aqr is an IP that underwent a low state in 2016 (decrease of the optical luminosity by 2 mag, from a normal state with V = 13.5 to a faint state with V = 15.5), from which it recovered slowly and steadily over a time scale of several months (Littlefield et al 2016). X-ray observations (Kennedy et al 2017) show that the mass-transfer rate decreased by about one order of magnitude.…”

Dwarf nova outbursts in intermediate polars

“…FO Aqr is an intermediate polar that has never shown outbursts, and can therefore be considered as steady, but was observed to enter a low state in 2016 (more than 2 mag decrease), from which it gradually recovered between May and July 2016 (Littlefield et al 2016). The partial eclipse observed during the low state is narrower and shallower than in the high state, indicating that an accretion disc is still present, albeit smaller than during the high state.…”