AAVSO Alert Notice 795 announces an observing campaign on the intermediate polar FO Aqr in support of upcoming HST observations. Please see the notice for details and observing instructions.
There are threads for this campaign under the following AAVSO forums:
Please subscribe to these threads if you are participating in the campaign so you can be updated. Join in the discussion or ask questions there!
Many thanks, and Good observing,
Elizabeth O. Waagen, AAVSO HQ
Thank you all for adding FO Aqr to your observing list. We have sent observation of the past few days to Space Telescope and they are happy with the brightness so far. I have also observed FO Aqr with the Notre Dame campus telescope despite rather poor weather in the mid-west. Good observing!
It is crunch time for FO Aqr! Hubble will visit FO Aqr twice this week and the Space Telescope Science Institute is requiring photometry to show that it is not unusually bright. For the HST observations to proceed, we need magnitudes measured tonight and reported by 8am Wednesday morning (Eastern Time). So if you do observe FO Aqr, please report the results promptly so we can get those numbers to STScI before their go/no-go decision meeting Wednesday morning.
The second visit is Friday, so we need to repeat this on Thursday night with magnitudes reported to STScI by 8am Friday.
It will be raining here in South Bend, Indiana tonight, so I can't help. But I hope to contribute Wednesday and Thursday nights when it is expected to clear off.
FO Aqr (JD)
2459880.47961 13.278 0.008 V
2459880.48409 13.592 0.008 B
2459880.48817 13.222 0.009 R
2459880.51928 13.341 0.007 V
2459880.52373 13.612 0.006 B
2459880.52781 13.322 0.010 R
STScI has clear the first visit to FO Aqr based on your observations this past week. Whew!
The first HST visit will go ahead tonight: Start time: 27 Oct 2022 02:40:47; End time: 27 Oct 2022 06:45:02
It may clear off in Indiana and I will try to do some simultaneous time-series observations.
****The second visit needs a go/no-go decision Friday morning, so additional photometry tonight and Thursday night are needed to clear those UV observations with HST.
Thank you all for your help on getting the first visit approved.
I was able to get a few hours of photometry from 2459879.69 - 2459879.80 (Wed night to Thur morning). A visual observation of the variable light curve (jpeg below) shows a period of ~20 min and amplitude of ~100-200 mmag, which are in rough agreement with the primary spin period. More importantly here, the mean G-band (Gaia green) magnitude is observed as 13.525 +-.012 (Gaia catalog value is G=13.5232). So all appears normal. I will attempt the same tonight. But it's already about 10am ET by the time the sun comes up here in Arizona.
Yes, the around orbital phases of 0.1 to 0.5 the spin pulse is strong with an amplitude of 0.2 mag. The phases between 0.5 and 1.0
tend to have a lower amplitude and less regularity.
I had hoped to get several sets of images tonight, but after the first, clouds rolled in. Hope this is better than nothing
Band Mag Error JD UT
V 13.672 (0.015) 2459880.58105 2022/10/28 01:56:43
B 13.811 (0.041) 2459880.58961 2022/10/28 02:09:02
The observations have been uploaded via WebObs.
Thur night to Friday morning (2459880.568 - 2459880.802) showed the same irregular spin variations with a mean Gmag=13.395 +-.012.
STScI has approved the second COS visit to FO Aqr scheduled for tonight (Friday night/Saturday morning). Thank you all for your observations that made this approval possible.
I downloaded the 3-orbit spectrum from the first visit and there are emission lines there...success! We'll have to see how the spectrum varies over the binary orbit.
The second (and final) visit will occur between:
Start time: 29 Oct 2022 05:27:30 End time: 29 Oct 2022 07:56:32
FO Aqr will be too low for me at the start of this window to get any simultaneous observations from South Bend. It might be up for a bit for people further west.
Last night I gathered 4 more BV observations of FO Aqr. They were space approximately 25 minutes apart, mainly because I need to stack 6 two minute exposures to get a reasonable SNR with my equipment.
To my surprise, the B-V color index changed dramatically between the 4 exposures. My first two had B-V positive and then my last 2 have a negative B-V. The star was brighter in B while the star was in the ascending part of its light curve (in V).
Here is the plot:
Is this expected? I don't think there was any errors in my measurements but you never know.
My observer code is TBOB if you want to look at them.
I am not sure there has been many observations of color variations in FO Aqr. It varies quickly, so you need near-simultaneous two-filter observations to be sure the color change is real and not a result of an over-all change in brightness. I've done some short exposures while alternating B and I filters (it drives the filter wheel crazy!) and it does look like the peaks of the spin pulses are more blue than the minima. And, what you may be seeing, is that the phases around the partial eclipse look more blue than the rest of the orbit.
It is still and interesting system. I hope our ground observations combined with the HST UV spectra will help figure it out.
I see what you mean…
I see what you mean. Looking at the light curves, it appears there is some sort of ~20 minute periodicity. With things changing that fast, my B readings are not closely related to the V ones so they can't be directly compared. I could probably get away with 1-2 minute exposures between V & B and still have a barely acceptable SNR. I might give that a try to see what happens. If nothing else, it is good practice for me.
Yes, the white dwarf spin period is 20.9 minutes and that dominates the light curve much of the time. Doing 1 minute exposures and alternating filters might work to get a color. Averaging the B mags on each side of the V mag will approximate the brightness at the time of the V exposure. That might help too.
As a follow-up to the discussion started by Bill Tschumy (TBOB), here are my measurements. The time interval between the B and V images is about 77s and between V and R 38s. The images in the different filters are not obtained simultaneously so the color values obtained are not accurate. Perhaps we can say that FO Aqr is always brighter in V than in B and that the index (V-R) is small and of the same order as the measurement errors. It would be interesting to make these measurements with a simultaneous multi-colour imager like the MuSCAT3 imager installed on the FTN in Hawaii but which operates in SDSS g'r'i'zs bands. We would therefore need to know the magnitudes in these spectral bands of the comparison stars. What do you think?
October 19 B-V=0.100 e= 0.054 V-R= - 0.082 e=0.048
October 21 B-V=0.245 e= 0.044 V-R= - 0.055 e=0.041
October 22 B-V=0.125 e= 0.062 V-R= 0.032 e=0.057
October 27 B-V=0.198 e= 0.053 V-R= 0.004 e=0.047
October 28 B-V=0.000 e= 0.050 V-R= 0.139 e=0.047
October 29 B-V=0.081 e= 0.044 V-R= - 0.017 e=0.040
October 31 B-V=0.084 e= 0.053 V-R= 0.031 e=0.048
All the best
The good news: HST successfully observed FO Aqr last night. The average UV spectrum looks great. Thank you all for your work in getting the observation approved by the COS safety folks at STScI.
The bad news: the spectrum from the first visit was not as nice as the second visit. I was pondering this problem when an email from STScI arrived telling me that the guide star did not achieve lock on the first visit and this may have compromised the data. Indeed, a comparison between the two spectra shows that the second visit had a 300 times higher count rate than the first visit.
I will send in a request to have the observations repeated. So I may be back one of these days asking for your help again.