FO Aquarii begins its 2018 observing season with its third low state in two years

ATel #11844; Colin Littlefield (University of Notre Dame), Robert Stiller (University of Notre Dame), Franz-Josef Hambsch (AAVSO / VVS / BAV / Center for Backyard Astrophysics), Benjamin Shappee (University of Hawaii), Thomas Holoien (Carnegie Observatories), Peter Garnavich (University of Notre Dame), Mark Kennedy (University of Manchester)
on 12 Jul 2018; 23:45 UT
Credential Certification: Colin Littlefield (clittlef@alumni.nd.edu)

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Photometry obtained by the All-Sky Automated Survey for Supernovae (ASAS-SN; Shappee et al. 2014, ApJ, 788, 48; Kochanek et al. 2017, PASP, 129, 104502) reveals that in mid-May, the intermediate polar FO Aquarii (FO Aqr) faded by 0.4 mag in the g band, consistent with a decrease in the binary's mass-transfer rate. In the linked figure, we plot the 124 g-band observations obtained by ASAS-SN between 2018 April 17-June 29, along with the best-fit sigmoid step-function model. Because FO Aqr has several large-amplitude, short-period variations in its light curve, the ASAS-SN points show considerable scatter with respect to the model; still, the fading behavior is readily apparent.

Although FO Aqr remained mired in its 2017 low state as it approached solar conjunction in 2018 January, the presence of a bright state during April and early May establishes that the 2018 faint state is not a continuation of the 2017 event.

FO Aqr has now experienced separate low states in 2016, 2017, and 2018. Prior to 2016, it had never been observed in one, despite a nearly century-long observational record (e.g., Garnavich & Szkody 1988, PASP, 100, 1522). The 0.4-mag depth of the 2018 low state is considerably less than its counterparts in 2016 (~2 mag; Littlefield et al. 2016, ApJ, 833, 93) and 2017 (~1 mag; Littlefield et al., in prep.). Nevertheless, Lomb-Scargle periodograms of our time-series photometry of the 2018 low state --- obtained with the University of Notre Dame's 80-cm Sarah L. Krizmanich Telescope and co-author Hambsch's 40-cm optimized Dall-Kirkham telescope --- show greatly elevated power at the spin-orbit beat frequency when compared with the system's high-state power spectrum (Kennedy et al. 2016, MNRAS, 459, 3622), a change that was also present throughout both the 2016 and 2017 low states (Littlefield et al., in prep.). The observed power spectrum implies a direct interaction between the white dwarf's magnetosphere and the accretion flow (Ferrario & Wickramasinghe 1999, MNRAS, 309, 517).

The high state in early 2018 was unusually faint by FO Aqr's historical standards. In ASAS-SN V-band photometry from 2013, 2014, and 2015, the system's average magnitude was stable at V = 13.57 +/- 0.01. By contrast, in a high state sandwiched between the 2016 and 2017 fading episodes, the average magnitude was V = 13.74 +/- 0.02, and during the bright interlude in early 2018, it was just V = 13.92 +/- 0.04. Thus, FO Aqr's high states are becoming progressively fainter, suggesting that even after a fading event has concluded, the system's mass-transfer rate does not return to its original level.

ASAS-SN g-band light curve