AAVSO Alert Notice 644 announces an observing campaign on the intermediate polar FO Aqr. Please see the notice for details and observing instructions.
Many thanks, and Good observing,
Elizabeth O. Waagen, AAVSO HQ
Late 2016 I set up the CCD chart I stlll use for FO Aqr, id = X16927BD. I just pulled it up from the Variable Star Plotter as:
and it still looks OK, no problems among the comp stars. So at worst you may consider starting with that.
No charge :)
jji - your suggestion was on target. My problem was the Special Chart was checked. I was getting M11 charts for doing transform coeficients a several days ago and forgot to uncheck Special Chart.
Thank you for observing FO Aqr. During the past ~20 days, FO has shown at least four ~0.3-mag flares in its light curve, during which the 20.9-minute spin pulse has become extremely strong. Outside of the flaring episodes, the spin pulsation has been comparatively weak or absent, possibly attributable to an increase in the strength of the system's 22.5-min beat period. This suggests that FO's accretion mechanism is changing during these flares.
During the flares, the system briefly reaches its brightness from before the start of the current low state (V~13.9), implying that the low state might almost be over. Thus, I suspect that the recent flares might be related to the reappearance of the system's accretion disk. During its bright state, FO Aqr contains a Keplerian accretion disk, but Hameury & Lasota (2017) predict that the disk dissipates during a low state. If they are correct, I would expect a transitional state near the end of a low state as the system reestablishes its accretion disk, and that might be what's responsible for these oscillations in the light curve.
Regardless of the cause of the system's recent behavior, it's important to continue monitoring it. Keep up the good work!
The results from the night of 2018-08-22 are submitted. Indeed the 0.3 to 0.5 mag flares with about 21 min period are clearly detectable. I will continue with observations.
Thank you for the outstanding coverage of FO Aqr during the past six weeks. Since approximately Aug. 18, FO Aqr has been in a bright state at about V = 13.9. During this time, the WD spin period at 20.9 minutes has dominated the light curve, and the strong 22.5-minute beat period observed throughout the low state has been weak or absent in most light curves. Consequently, it seems that this year's low state has ended, and intensive observations are no longer necessary.
The attached light curve shows the average magnitude of FO Aqr in light curves submitted by various observers, and you can see a series of flares and dips in the first half of August. The horizontal dashed line represents the system's brightness before the low state. The different marker styles correspond to different observers. The errorbars are derived from simulations involving the Kepler K2 light curve of FO Aqr, and their sizes are dictated solely by the length of the underlying time series. This is because longer time series do a better job of averaging over both the periodic and aperiodic variability in FO Aqr, giving a better sense of the system's overall brightness.
Even in the absence of an active observing campaign, FO Aqr is a unique and dynamic system, and there is a lot of merit in observing it occasionally. There's no telling when its next low state will begin, how deep that low state will be, or how long it will last. A very deep low state (fainter than V = 15) would be of particular interest, as it would present an opportunity to detect the donor star in spectra for the first time ever. Even if FO stays bright, your observations of the 20.9-minute spin pulsations provide direct measurements of the unpredictable evolution of the white dwarf's spin period, a subject that has received significant attention in the literature. The bottom line is that it will be scientifically fruitful to check in on FO Aqr from time to time.
My colleagues and I are grateful for all of the data that you've contributed since 2016. I'd like to offer a quick update on FO Aqr as the 2020 observing season begins. Here are the key points:
- Intensive observations aren't necessary unless FO Aqr shows previously unobserved behavior --- for example, an especially deep fading (> 2 mag). I'll post a notice if that happens.
- An occasional time series (~1 per month by a few observers) would be very helpful, particularly if FO Aqr is in its bright state. The goal would be to monitor the changing spin period of the white dwarf. (Details are below, for anyone interested.)
Details about monitoring the spin period: One of the most basic properties of an intermediate polar like FO Aqr is the white dwarf's spin period, the evolution of which is closely tied to theories about angular-momentum transfer. Monitoring the spin period lets us test those theories.
The evolution of the white dwarf's spin period in FO Aqr has been much more complex and rapid than in other intermediate polars. From the early 1980s - 1987, the white dwarf's rotation was slowing down, but during the following quarter-century, it began to speed up. Then, only several years ago, it started slowing down again. No other intermediate polar has showed such complex changes to its spin period.
To keep track of the spin period, researchers need observations of FO Aqr's "spin pulse," which is a periodic hump in the light curve that repeats every 20.9 minutes (the spin period of the WD). For each pulse, we measure the time when the pulse reached its maximum brightness, and we repeat this procedure for all pulses observed in an observing season. If you combine these timings with those from other studies, you can determine how the spin period has evolved over the baseline of observations.
If you'd like to contribute to this effort, PLEASE TRY TO OBTAIN ONE OR TWO LONG, UNINTERRUPTED TIME SERIES OF FO AQR EACH MONTH. It's critically important to ensure that your computer's clock is accurate so that the CCD timestamps are as well. Ideally, the observations would cover most (if not all) of the 4.85-hour binary orbit. Unfiltered or V would be preferred. Please note that when FO Aqr is in its bright state, the spin pulse has a large amplitude (~0.4 mag) and is the dominant feature in the light curve; however, in the low state, the spin pulse is very difficult to identify.
Dr. Colin Littlefield has changed the current observing cadence he would currently like for FO Aqr. Please see his post above this one, particularly the bold text in the last paragraph of it. I also added this change of instructions to the Note above the text of Alert Notice 644 and to the FO Aqr entries in the Target Tool.
FO Aqr is currently hovering around V~14. In the past, this has been the approximate threshold at which it has dropped into a low state and developed strong periodicities at 22.5 min and 11.3 min. When FO is above V~14, the light curve shows a dominant periodicity at 20.9 minutes due to the rotation of the WD and matter trapped in its magnetosphere. In the most recent AAVSO data, the 11.3-min signal is beginning to appear, but the 20.9 min signal is still dominant.
Based on data contributed by our 43 AAVSO co-authors, my colleagues and I proposed in a recent paper that this abrupt change at V~14 corresponds with the dissipation of the accretion disk.
Although intensive observations are not necessary at this time, it would be wonderful if observers could maintain their current coverage. As a reminder, we'd like time-series photometry at a reasonably fast cadence (< 60 sec per image) that covers as much of the 4.85-hour binary orbit as possible. I will monitor the AAVSO observations as they come in, and if FO Aqr starts to show behavior not observed during the previous low states (such as a complete cessation of accretion), I might humbly ask for more intensive observations.
P.S. Just to clarify the most recent post: I don't hold a PhD.