Alert Notice 598: Intermediate polar FO Aqr fading and photometry needed now

September 6, 2017:  Dr. Colin Littlefield (University of Notre Dame) and colleagues have requested AAVSO assistance in monitoring the intermediate polar cataclysmic variable FO Aqr beginning immediately. This campaign is similar to the one on FO Aqr requested by Dr. Littlefield and colleagues in July 2016 (AAVSO Alert Notice 545).

Dr. Littlefield and co-authors Gordon Myers (AAVSO), Richard Sabo (AAVSO), Peter Garnavich (Notre Dame), and Mark Kennedy (University College Cork) recently published ATel #10703, and write therein that: "The intermediate polar FO Aqr, which in 2016 underwent the first low state in its observational history (Littlefield et al. 2016, ApJ, 833, 93), has faded by nearly 0.7 mag since 2017 August 10, implying a decrease in the system's accretion rate."

Good coverage of the current fading event - only the second observed in this system - is extremely important.

In an email Dr. Littlefield writes: "As with the previous campaign, we would like observers to obtain nightly photometric time series of FO Aqr. In order to maximize their usefulness to us, the observations would need to be either V-filtered or unfiltered with a V zeropoint (i.e., unfiltered but using the V magnitude of a comparison star when performing photometry, CV). We would like observers to use a cadence of 60 sec/image or faster, even if this means observing unfiltered, and we would prefer that each time series last at least an hour (preferably a minimum of 2 hrs). Our hope is that the AAVSO could observe FO Aqr until either (1) the end of the current faint state or (2) solar conjunction, which for FO Aqr is in January." Visual observations are also welcome.

He continues: "Because last year's fade began during solar conjunction, the drop to minimum went almost entirely unobserved. If this is the start of another deep low state like the 2016 event, it would be the first opportunity to study how FO Aqr's accretion processes change during the drop to minimum. For example, AAVSO observations of the current fade by observers Gordon Myers and Richard Sabo have already provided important evidence of a correlation between FO Aqr's overall brightness and the mechanism by which material accretes onto the system's white dwarf.

"We had 16 AAVSO co-authors on our paper about the 2016 low state, and if observations from the new campaign lead to another publication, we would be pleased to offer co-authorship to AAVSO observers who contribute data used in our analysis."

Observations in the AAVSO International Database on 2017 September 2.6251 UT show FO Aqr at CV = 14.607 +/-0.010 (MGW, G. Myers, Hillsborough, CA). AAVSO data show that during the 2016 fading episode it was as faint as CV = 15.6. FO Aqr's brightness in its high state is V~13.4.

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 observing campaign is being followed on the AAVSO Campaigns and Observing Reports online forum at

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


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