FO Aqr 2016 low state and recovery (2015-16 seasons)
Observed: 10 Jul, 31 Oct, 6, 8 Nov, 26 Dec 2015, 28 Jun, 3, 9, 18, 28 Jul, 3, 12, 13, 22, 25 Aug, 1, 6, 28 Sep, 30 Nov, 8, 10, 18, 28, 29 Dec 2016
Michel Bonnardeau
14 July 2016
Updated 18, 29 Aug, 5, 11 Sep, 15 Dec 2016, 3 Jan 2017 with more data
6 Jan 2017 O-C analysis, 20 Feb 2017, 4 Apr 2019
Abstract
In 2016, the intermediate polar FO Aqr has been observed in an unusual low state and slowly recovering. The O-C are compared with those from my previous observations.
Introduction
FO Aqr is a cataclysmic variable star, that is a binary star with an accreting white dwarf. The white dwarf is magnetic, funneling the accretion, and FO Aqr is classified as an Intermediate Polar. The orbital period is Porb=4.849 hours and the spin period of the white dwarf is Prot=20.9 minutes.
I have been monitoring FO Aqr every season since 2004.
Observations
The observations were carried out with a 203 mm Schmidt-Cassegrain telescope, a Clear filter, and a SBIG ST7E camera (KAF401E CCD), mostly red sensitive. The exposure durations were 60 s. 385 useful images were obtained in 5 nights in 2015, 2635 images in 19 nights in 2016.
For the differential photometry, the comparison star is GSC 5803-398 (UCAC4 409-138153 with V=10.921 mag). The check star is UCAC4 409-138161.
Season 2015
The magnitudes are the differences with the comparison star.
Red: FO Aqr, Blue: the check star shifted by +0.3 mag. The error bars
are the quadratic sum of the 1-sigma statistical uncertainties on the variable
and of the comparison star.
There was an interruption to observe an occultation.
Saison 2016
FO Aqr is dimmer by about one and a half mag:
The check star is shifted by +1.6 mag.
The data are noisy because of the Moon.
Averages
With the magnitudes averaged over each night:
The error bars are the standard deviations over each night.
A close up for 2016:
This low state was reported by Littlefield et al (ATEL #9216). They also discovered a 11.26-minute periodicity (ATEL #9225). These data are also used in Littlefield et al ApJ 833 93.
O-C analysis
In my study of FO Aqr for the seasons 2004 to 2015 (IBVS 6181), the data were fitted, season by season, with an analytical function featuring a constant unmodulated amplitude and modulated amplitudes. This allowed filtering out the irregularities of the amplitudes and the intermittent sideband signals.
For the 2016 season, the amplitudes of the unmodulated part and of the modulated parts cannot be taken as constant, as the system evolves from a faint state to a normal state during the whole season.
So I proceed the same way I did for my
study of BG CMi for the seasons 2005 to 2016
(IBVS 6168):
I select 47 pulses from the light curves, strong and well shaped,
and I compute the O-C from the 2004-2015 ephemeris. The resulting diagram is:
To make a comparison, I did the same with 10 pulses I observed in 2015.
The resulting diagram is:
On the O-C diagram for 2016, there is a lot of dispersion at the beginning of the season. This is due to the presence of sideband signals and the pulses are not all due exactly to the spin modulation (Littlefield et al, ATEL #9225).
I plot for the 2016 and the 2015 seasons the average O-C and
the average BJD of the pulses along with the O-C of my 20104-15 study:
Small blue dots: O-C for 2004 to 2015; Open blue circle: average from
10 pulses in 2015; Red: average from 47 pulses in 2016.
Technical notes
Telescope and camera configuration.
Computer and software configuration.
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