A new fading event is underway in FO Aqr

ATel #10703; Colin Littlefield (Notre Dame), Gordon Myers (AAVSO), Richard Sabo (AAVSO), Peter Garnavich (Notre Dame), Mark Kennedy (University College Cork)
on 3 Sep 2017; 21:22 UT
Credential Certification: Colin Littlefield (clittlef@alumni.nd.edu)

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The intermediate polar FO Aqr, which in 2016 underwent the first low state in its observational history (Littlefield et al. 2016, ApJ, 833, 93), has faded by nearly 0.7 mag since 2017 August 10, implying a decrease in the system's accretion rate.

To investigate FO Aqr's behavior, we obtained a total of 19 photometric time series using three instruments: the University of Notre Dame's 80-cm Sarah L. Krizmanich Telescope (SLKT) and separate 43-cm reflectors operated by co-authors Myers and Sabo (denoted in the linked figure as MGW and SRIC, respectively). Each time series was unfiltered with a V zeropoint to shorten the exposure time. To estimate the overall brightness of FO Aqr in these data, we found the median magnitude of each time series. Since FO Aqr's famous spin modulation can produce a 0.5-mag pulsation every 21 minutes, a multi-hour time series helps to isolate the system's overall brightness from its fast variability.

We supplemented our time-series observations with photometry from the All-Sky Automated Survey for Supernovae (ASAS-SN: Shappee et al. 2014, ApJ, 788, 48; Kochanek et al. 2017, PASP submitted, arXiv:1706.07060). However, because the ASAS-SN data generally consist of small groups of closely spaced, 90-second exposures taken every few nights, they are susceptible to contamination from short-term variability; thus, they show more scatter with respect to the overall trend than do the median magnitudes of the time series.

In the linked figure, we plot a light curve of FO Aqr, superimposing a simple linear fit to the current fading event. Although there are significant and erratic deviations from the average fading rate of 0.029 +/- 0.003 mag per day, the system had dropped to V ~ 14.4 as of Sept. 2. For comparison, the median magnitude of FO Aqr in 498 ASAS-SN observations from 2012-2015 was V = 13.54, and in 159 measurements between the end of the 2016 low state and the start of the current fading episode, it was V = 13.74. Other than the ongoing fade and the 2016 event, FO Aqr's overall brightness has remained stable, so the current ~0.7-mag fade is unusual behavior for this system.

The brightness variations are correlated with significant changes in the system's power spectrum. When FO Aqr is bright, the white dwarf's spin frequency is the dominant signal, but in the five faintest light curves in our dataset, there is considerable power at the spin-orbit beat frequency and its next harmonic (the 'double-beat' frequency). The presence of power at the beat and double-beat frequencies indicates an interaction between the WD's magnetosphere and fixed structures within the binary rest frame. Both of these frequencies were detected for much of the 2016 low state, and they were interpreted as evidence of direct interaction between the accretion stream and the white dwarf's magnetosphere.

If FO Aqr's current faintness is a harbinger of another deep low state, intensive coverage of the decline might clarify lingering questions from the 2016 event, such as whether FO Aqr's disk fully dissipates near the minimum of a low state (Hameury & Lasota 2017, A&A submitted, arXiv:1707.00540). Follow-up X-ray observations using Swift will take place over the coming weeks, and we urge further photometric and spectroscopic observations.

Light Curve