For nearly four decades following its discovery, the intermediate polar FO Aquarii (FO Aqr) was observed exclusively in a state of high mass transfer. However, in each year between 2016 and 2019, it experienced low-accretion states that were thought to be unprecedented until the identification of pre-discovery low states in photographic plates from 1965, 1966, and 1974. By combining an extensive set of time-series photometry with data from previous studies, we calculate an updated white-dwarf spin ephemeris and establish that the 2016-2019 low states occurred shortly after the spin period of the white dwarf began to increase, after having decreased for a quarter century. If this quarter-century interval is representative of typical spin-up and spin-down episodes in FO Aqr, then the newly identified low states from the mid-20th century would have likely occurred during the previous spin-down episode of the white dwarf. Conversely, no low states have been observed during a known period of spin-up. We find that the power generated by the spin-down of the white dwarf is ~0.5-1 solar luminosity, and while we do not know where this energy is being deposited, we speculate that it might drive an outflow. We explore how the correlation between FO Aqr's low states and the spin-down of the white dwarf can be more rigorously tested as well as its implications for our understanding of both FO Aqr and the class of intermediate polars.
American Astronomical Society Meeting Abstracts #235
- Pub Date:
- January 2020