Doubling Down: A Dominant, 11.26-Minute Photometric Period in FO Aqr
ATel #9225; Colin Littlefield (Notre Dame), Erin Aadland (Minnesota State), Peter Garnavich (Notre Dame), Mark Kennedy (University College Cork)
on 8 Jul 2016; 05:59 UT
Credential Certification: Colin Littlefield (clittlef@alumni.nd.edu)
Subjects: Optical, Request for Observations, Binary, Cataclysmic Variable, Variables
We obtained time-resolved photometry of the intermediate polar FO Aqr during its ongoing low state (ATEL #9216) on 2016 July 2, 6, and 7. We observed at a cadence of 8 seconds using the University of Notre Dame's 80-cm Sarah L. Krizmanich Telescope. All data were unfiltered with a Johnson V zeropoint. A representative light curve, along with a Lomb-Scargle periodogram of the combined dataset, is available at this link.
The most surprising feature of our data is the presence of a very prominent 11.26-minute periodicity, corresponding with one-half of the beat period between the spin and orbital periods. To check the stability of this period, we computed a power spectrum for each individual time series, and the 11-minute signal was by far the strongest period in all three light curves. Interestingly, there is little evidence of the beat period itself.
The 20.9-minute spin period of the white dwarf (WD) is also conspicuous in the power spectrum, although the 11-minute signal is much stronger. For comparison, during FO Aqr's bright state, the spin modulation of the WD was the dominant source of photometric variation on timescales of less than the orbital period, and the 11-minute signal was comparatively weak (Kennedy et al. 2016, MNRAS, 459, 3622).
Such a strong signal at half of the beat period suggests an interaction between the rotating WD and structures fixed within the binary rest frame. For example, if the accretion stream were to overflow the disk and travel to the WD's magnetosphere, it is conceivable that the overflowing stream would be channeled onto alternating magnetic poles as a consequence of the WD's rotation with respect to the stream. As a result, the stream might alternate between magnetic poles twice each beat period, possibly producing the observed signal.
Regardless of the exact cause of the 11.26-minute period, the reduced accretion rate has clearly resulted in dramatic changes in the system's accretion processes. FO Aqr merits continued photometric and spectroscopic monitoring.
Power spectrum and a representative light curve
