Down with the King: FO Aqr in an Extended Low State

ATel #9216; Colin Littlefield (Notre Dame), Peter Garnavich (Notre Dame), Erin Aadland (Minnesota State), Mark Kennedy (University College Cork)
on 6 Jul 2016; 23:45 UT
Credential Certification: Colin Littlefield (clittlef@alumni.nd.edu)

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The cataclysmic variable FO Aquarii, the so-called "king of the intermediate polars," is presently in an unprecedented and prolonged faint state. Normally brighter than V = 14, FO Aqr is currently at V ~15 and was as faint as V~15.7 at the start of 2016 May. The faint state began at an unknown time between 2015 Dec. 18 (V = 13.71, S. Dvorak, AAVSO) and 2016 May 6 (V ~ 15.6, our measurement).

To investigate this behavior, we combined photometry from the Catalina Real-Time Sky Survey (CRTS; Drake et al. 2009, ApJ, 696, 870), the 1.8-m Vatican Advanced Technology Telescope (VATT), and the University of Notre Dame's 80-cm Sarah L. Krizmanich Telescope (SLKT). The VATT and SLKT data were unfiltered with a Johnson V zeropoint. We also downloaded the AAVSO observations of FO Aqr made after 2015 Oct. 1, all of which were V-filtered CCD observations contributed by Shawn Dvorak (observer code DKS). Together, these datasets show a gradual, quasi-linear recovery since early May, with FO Aqr brightening at an average rate of approximately -0.01 mag/day. At the time of writing, FO Aqr had brightened to V ~ 15, still well below its normal brightness. A light curve of the data is available here.

Although FO Aqr was recently observed by the K2 mission (Kennedy et al. 2016, MNRAS, 459, 3622), the Kepler observations ended before the start of the current low state.

Prior to this faint state, FO Aqr had been in an extended bright state, implying a stable rate of mass transfer. A study using the Harvard Plate Collection showed no evidence of faint states between 1923 and 1953 (Garnavich and Szkody 1988, PASP, 100, 1522). More recently, light curves from the CRTS and the AAVSO show the system to be consistently brighter than V~14, with no indication of prolonged low states. Additionally, the AAVSO's International Variable Star Index lists FO Aqr's variability range as 12.7 (V) -14.2 (V), further underscoring the exceptional nature of the current low state. Indeed, we are unaware of any published measurement of this system in a faint state.

Continued monitoring of this event, particularly by AAVSO observers, would provide insight into the ongoing low state as well as the effect of the diminished mass transfer on FO Aqr's magnetic accretion processes. A spectroscopic study would likely prove to be especially fruitful.

We acknowledge with thanks the variable star observations from the AAVSO International Database contributed by observers worldwide and used in this research. We are particularly grateful to Shawn Dvorak for his sustained observations of FO Aqr.

Light Curve of Low State