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Alert Notice 545: FO Aqr time-series observations requested

July 13, 2016: Dr. Colin Littlefield (University of Notre Dame) and colleagues Drs. Peter Garnavich (Notre Dame), Erin Aadland (Minnesota State), and Mark Kennedy (University College Cork) have requested AAVSO assistance in monitoring the intermediate polar cataclysmic variable FO Aqr beginning immediately.

Dr. Littlefield, who with his colleagues recently published ATel #9216 and #9225, writes:

"This system is in a faint state for the first time in its observational record, implying a dropoff in the mass-transfer rate. AAVSO observations contributed by Shawn Dvorak [the only observer following FO Aqr at the time] were particularly helpful in detecting this low state. Since early May, the system has recovered to V~15, but it is still well below its normal brightness. In addition, our time-series photometry shows a very strong 11.26-minute photometric period. By contrast, during its bright state, FO Aqr's light curve is dominated by a 20.9-minute period, corresponding with the spin period of the white dwarf. We interpret our observations as evidence that the system's accretion processes have changed dramatically as a result of the reduced mass-transfer rate.

"We have...determined that...[the 11.26-min] periodicity is dependent on the orbital phase of the binary. The 11.26-min period is dominant for about half of the orbit, but for the other half, a 22.5-min period is stronger. AAVSO observers can help us study both of these periods as well as their dependence on the orbital phase. We are particularly interested in any changes in this behavior as the system continues to brighten.

"...Time-series photometry of FO Aqr [is requested] in order to better study the evolution of the 11.26-minute period as the system rebrightens. Unfiltered photometry reduced with a V zeropoint would be the most useful to us... A cadence of less than 60 seconds per image is important, given the brevity of these periods (especially the 11.26-min period). Long, uninterrupted time series would be especially valuable, but even a short time series would be useful in terms of monitoring the 11-minute period. Also, it's critical that observers ensure that their computer clocks are accurate to within a few seconds."

Littlefield reports that as of 2016 July 12-13, FO Aqr was "still much fainter than usual, at V~14.93 (unfiltered with V zeropoint). The data remain consistent with a gradual rebrightening at an average rate of about -0.01 mag/day." FO Aqr's normal brightness is V~13.4.

"We would like the time-series observations to continue until the system returns to its normal brightness level or until it gets too close to the Sun to be observed (whichever comes first)."

Coordinates (2000.0): R.A. 22 17 55.38  Dec. -08 21 03.8

Charts: Charts for FO Aqr with an expanded comparison star sequence may be created using the AAVSO Variable Star Plotter (VSP). For observers who have been following FO Aqr as part of the CBA program, the comparison stars used for that program have been retained.

Please submit observations to the AAVSO International Database using the name FO AQR.

This campaign is being monitored on the AAVSO Observing Campaigns webpage at https://www.aavso.org/observing-campaigns.

This AAVSO Alert Notice was compiled by Elizabeth O. Waagen.

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