Alert Notice 644: FO Aqr monitoring requested in faint-state study

Nightly monitoring of FO Aqr is requested throughout the month of October 2022. Many thanks.  -  Elizabeth O. Waagen, 3 October 2022

Please continue observing FO Aqr according to the revised observing cadence given in the next paragraph.  -  Elizabeth O. Waagen, 1 February 2021

Note: As of June 2020, the REQUESTED OBSERVING CADENCE IS CHANGED. Please obtain at least one long light curve (time series) per month per observer while FO Aqr is in its high state. This cadence will help to enable keeping track of the rapidly changing spin period.  The cadence should be higher if FO Aqr goes into its low state.  -  Elizabeth O. Waagen, 21 June 2020

As of May 2020, a nightly snapshot once a week while FO Aqr is in its high state is requested. Higher cadence is requested when it goes into its faint (low) state.  -  Elizabeth O. Waagen, 20 May 2020

July 27, 2018:

Dr. Colin Littlefield (University of Notre Dame) writes:

"In both 2016 and 2017, my colleagues and I requested AAVSO campaigns on the intermediate polar FO Aquarii to study its first-ever faint states. In both campaigns, we were delighted with both the quality and quantity of the data that we received from AAVSO observers [AAVSO Alert Notices 545, 598).

"FO Aqr has entered its third low state in two years (ATEL 11844), and on behalf of my colleagues, Dr. Peter Garnavich (University of Notre Dame) and Dr. Mark Kennedy (University of Manchester), I would like to request a renewed campaign on it.

Science overview

"FO Aqr is an interacting binary star, featuring a magnetized white dwarf that accretes from a low-mass companion star. For almost all of FO Aqr's observational history, the accretion flow has formed an accretion disk, the inner region of which is truncated by the WD's magnetic field. Gas from the inner rim of the disk flows along the WD's field lines until it accretes onto the WD, producing a 0.5-mag pulse at the WD's spin period (P_spin = 20.9 min).

"During a low state, the mass-transfer rate decreases, depleting the disk to the point that it might even disappear altogether (Hameury & Lasota 2017, A&A, 606, 7).  Before 2016, FO Aqr had never been seen in a low state, despite nearly a century of observations. During separate low states in 2016 and 2017, however, the system faded by approximately 2 mag and 1 mag, respectively. In each of these low states, the amplitude of the spin pulse decreased, and periodicities at P = 22.5 min and P = 11.3 min became prominent in the light curve. The strength of these two periodicities indicates that in both low states, a significant fraction of the accretion flow directly impacted the WD's magnetosphere without passing through an accretion disk.

"AAVSO observations enable us to identify which periodicites are present in the light curve, and from that information, it is possible to draw conclusions about FO Aqr's accretion mechanisms. In particular, we are interested in using AAVSO data to test the hypothesis by Hameury & Lasota (2017) that FO Aqr's disk dissipates if a low state is deep enough.

Details of requested observations

"As in previous years, we request time-series photometry of FO Aqr. The system has a number of short-period variations (particularly at P = 22.5 min, 20.9 min, and 11.3 min) superimposed on its 4.85-hour orbital period, so we prefer cadences faster than ~60 seconds per image if possible. Either V-band or unfiltered with a V zeropoint would be ideal.

"Regarding the frequency of observation, we would like each observer to try to obtain at least one time series per week if possible, with that time series lasting as long as possible. Although more frequent time-series observations would be very welcome and scientifically valuable, we believe that a target of one time series per week per observer would strike a reasonable balance between our science objectives and not overburdening observers.

"We would like the campaign to continue until the end of the current observing season for FO Aqr (late December) or until FO Aqr recovers from its current low state -- whichever comes first.


"Our policy is to offer co-authorship to observers whose data we use in our analysis. In our paper about the 2016 low state, 16 AAVSO observers were co-authors. Within the next month or so, we aim to submit a paper that analyzes the 2000+ hours of AAVSO time-series photometry from both 2016 and 2017, and we will soon extend offers of co-authorship to the amateurs who contributed data."

Coordinates (2000): R.A. 22 17 55.38  Dec. -08 21 03.8  (from VSX)

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

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

AAVSO Forums: FO Aqr is the topic of the AAVSO Campaigns and Observation Reports forum thread

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

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