Alert Notice 832: Monitoring of FO Aqr needed for HST observations

September 19, 2023

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Dr. Peter Garnavich (Notre Dame University) requests AAVSO observers' assistance in monitoring the intermediate polar cataclysmic variable FO Aqr in support of spectroscopic observations with the Hubble Space Telescope scheduled for 2023 September 27 and 29.

Calibrated CCD observations (V, B, R, CV, CR) are requested beginning immediately and continuing through October 6. A single observation per night or a few observations spread over a night will be most useful. A signal-to-noise ratio of 20 or better is needed (uncertainty of +/-0.05 mag or less). It is very important to reduce and submit observations as soon as possible.

Confirmation observations that show FO Aqr is below the brightness limit are required on the mornings of September 26 and September 28 (EDT) [September 25/26 and 27/28] so that HST can be cleared for the visits.

Dr. Garnavich writes: "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.

"The COS spectrograph on HST has a sensitive photon-counting detector that can be damaged if the source is too bright. As FO Aqr is known to be variable, NASA is requiring that FO Aqr be monitored for a week before the HST visit and that a confirmation of its brightness be made less than 24 hours before data-taking begins. There are two planned visits within 48 hours, and both will require confirming brightness estimates.

"Scientifically, it is also important to know the optical state of FO Aqr during the UV observations. So optical data taken to assure the COS team that their detector is safe, will also help us know if the spectra are representative of the current FO Aqr properties."

FO Aqr has a range of 12.7 - 15.7 V. The most recent observation in the AAVSO International Database shows it at magnitude 13.309 CV +/- 0.012 on 2023 Sep. 18.3011 UT (J.-F. Hambsch, remotely in Atacama, Chile).

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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