Abstract
Abstract The intermediate polar FO Aquarii (FO Aqr) experienced its first reported low-accretion states in 2016, 2017, and 2018, and, using newly available photographic plates, we identify prediscovery low states in 1965, 1966, and 1974. The primary focus of our analysis, however, is an extensive set of time-series photometry obtained between 2002 and 2018, with particularly intensive coverage of the 2016–2018 low states. After computing an updated spin ephemeris for the white dwarf (WD), we show that its spin period began to increase in 2014 after having spent 27 yr decreasing; no other intermediate polar has experienced a sign change of its period derivative, but FO Aqr has now done so twice. Our central finding is that the recent low states all occurred shortly after the WD began to spin down, even though no low states were reported in the preceding quarter-century, when it was spinning up. Additionally, the system’s mode of accretion is extremely sensitive to the mass-transfer rate, with accretion being almost exclusively disk-fed when FO Aqr is brighter than V ∼ 14 and substantially stream-fed when it is not. Even in the low states, a grazing eclipse remains detectable, confirming the presence of a disklike structure (but not necessarily a Keplerian accretion disk). We relate these various observations to theoretical predictions that during the low state, the system’s accretion disk dissipates into a non-Keplerian ring of diamagnetic blobs. Finally, a new XMM-Newton observation from a high state in 2017 reveals an anomalously soft X-ray spectrum and diminished X-ray luminosity compared to pre-2016 observations.
Highlights
Despite its stature as one of the most extensively studied intermediate polars (IPs), FO Aquarii continues to offer fresh insight into this class of object, even four decades after its discovery
The beat phase describes the relative orientation of the white dwarf (WD) with respect to the secondary, so if the data are split into small bins according to their beat phase, each bin will describe the system at one particular accretion geometry
Our spin ephemeris links all pulse timings obtained since 2002 and establishes that the recent fusillade of low states began shortly after the WD reverted to a spin-down state in 2014
Summary
Despite its stature as one of the most extensively studied intermediate polars (IPs), FO Aquarii (hereinafter, FO Aqr) continues to offer fresh insight into this class of object, even four decades after its discovery. The WD’s magnetospheric radius determines the mode of accretion onto the WD; a large magnetosphere will disrupt the accretion stream before it can circularize into a disk, while a small one will allow the formation of a disk whose inner region is truncated at the magnetospheric radius. Depending on the colatitude of the magnetic axis and the orbital inclination, the WD’s rotation can cause a periodic variation in the aspect of the accretion curtain as well as regular disappearances of the accretion shock behind the limb of the WD. These effects generate optical and X-ray pulsations, respectively, at the WD’s spin frequency (ω). Relatively few low states have been observed in IPs, with Swift J0746.3-1608 (Bernardini et al 2019) being one of the few examples
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