Alert Notice 795: FO Aqr monitoring requested now for HST observations

Note: Please monitor FO Aqr nightly through the end of the observing season.  -  Elizabeth O. Waagen, October 31, 2022

October 18, 2022

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Drs. Peter Garnavich and Colin Littlefield (University of Notre Dame) request AAVSO observers' assistance beginning immediately in monitoring the intermediate polar cataclysmic variable FO Aqr in support of upcoming HST observations scheduled for 2022 October 27 and 29.

Dr. Garnavich writes: "The campaign is critical for support of [upcoming] Hubble Space Telescope observations of FO Aqr. FO Aqr will be observed with the COS spectrograph, and there is concern that its detector could be damaged if FO Aqr is significantly brighter than normal (V~13.5 mag). An HST observation will be scrubbed if there is not a positive V observation within 24 hours of the scheduled time.

"There is an additional science purpose for these observations: FO Aqr has gone through a series of low states starting in 2016. This is what prompted the new observations of FO Aqr by HST. It is important to know the optical brightness of FO Aqr around the time of the UV spectra to help interpret the results.

"FO Aqr is a bright 'intermediate polar' where a cool secondary star is donating mass to a rapidly spinning, magnetic white dwarf. The spinning white dwarf is seen as strong, 21-minute periodic pulses over the 4.85 hour binary orbital period. Starting in 2016, FO Aqr went into a series of unprecedented low-states after a century of varying in a narrow range around 13th mag. The origin of the sudden change in habits may be related to the magnetic white dwarf that switched to slowing its spin rate around the time the fading episodes began. The energy from the spinning white dwarf may now be driving a wind or jet from the system. UV spectroscopy with HST was proposed to search for an outflow."

Coverage is requested to begin immediately and continue through November 1.CCD BVR photometry is requested; CV or CR will also be useful. DSLR photometry is not requested for this campaign. Dr. Garnavich continues: "Single observations or a few observations spread over a night are best. Quick reduction and notification are important to assure NASA that FO Aqr is not doing something dangerous. The nights of October 25/26 and October 27/28 are particularly important to get clearance for the [HST observations]. A signal-to-noise ratio of 20 or better (uncertainly of +/-0.05 mag or less) is needed."

FO Aqr has a V range of 12.7 - 15.7 V. The most recent observations from the AAVSO International Database show FO Aqr at magnitude 13.449 V +/-0.016  on Oct. 18.02307 UT (T. Arranz, Navas de Oro, Spain).

Coordinates (J2000):  R.A. 22 17 55.38   Dec. -08 21 03.8  (from VSX entry for FO Aqr)

Charts with comparison stars for FO Aqr may be created using the AAVSO Variable Star Plotter (VSP).

Please report observations to the AAVSO International Database using the name FO AQR. Reduce and submit observations as soon as possible after making them.

This Alert Notice was compiled by Elizabeth O. Waagen using information provided by Dr. Garnavich.


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