Authors: Colin Littlefield, Peter Garnavich, Mark R. Kennedy, Erin Aadland, Donald M. Terndrup, Grace V. Calhoun, Paul Callanan, Lyu Abe, Philippe Bendjoya, Jean-Pierre Rivet, David Vernet, Maxime Devogele, Michel Bonnardeau, Andre Debackere, Shawn Dvorak, James R. Foster, William Goff, Franz-Josef Hambsch, Barbara Harris, Gordon Myers, Peter Nelson, Velimir Popov, Rob Solomon, William L. Stein, Geoff Stone

Publication: The Astrophysical Journal, 2016, 833, 93

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, with an e-folding time of 116±4 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.

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