Hitting a New Low: The Unique 28 hr Cessation of Accretion in the TESS Light Curve of YY Dra (DO Dra)

Abstract

We present the Transiting Exoplanet Surveying Satellite light curve of the intermediate polar YY Draconis (YY Dra, also known as DO Dra). The power spectrum indicates that while there is stream-fed accretion for most of the observational period, there is a day-long, flat-bottomed low state at the beginning of 2020 during which the only periodic signal is ellipsoidal variation and there is no appreciable flickering. We interpret this low state to be a complete cessation of accretion, a phenomenon that has been observed only once before in an intermediate polar. Simultaneous ground-based observations of this faint state establish that when accretion is negligible, YY Dra fades to g = 17.37 ± 0.12, which we infer to be the magnitude of the combined photospheric contributions of the white dwarf and its red dwarf companion. Using survey photometry, we identify additional low states in 2018–2019 during which YY Dra repeatedly fades to—but never below—this threshold. This implies relatively frequent cessations in accretion. Spectroscopic observations during future episodes of negligible accretion can be used to directly measure the field strength of the white dwarf by Zeeman splitting. Separately, we search newly available catalogs of variable stars in an attempt to resolve the long-standing dispute over the proper identifier of this system.

The orbital period versus absolute magnitude relationship of intermediate polars: implications for low states and outbursts

“…The deepest part ( ∼2 mag in the V band) of its low state is interpreted by Covington et al ( 2022 ) as due to cessation of accretion on to the white dwarf. This is corroborated by the non-detection in X-rays during a 55 ks NuSTAR observation (Shaw et al 2020 ), and by a low-state TESS light curve in which only the orbital modulation (via the ellipsoidal modulation) was detectable (Hill et al 2022 ). Similarly, accretion may have completely stopped in V1025 Cen at its lowest flux (Covington et al 2022 ).…”

TESS and ground based photometry of magnetic CVs

“…At much lower magnetic field strength of IPs, accretion rarely if ever stops as it does frequently in most polars [14]. The IP, YY Dra often known as DO Dra, was found to undergo occasional and very brief low accretion states due to a cessation of mass transfer [15]. The Interactive Magnetic Valve Model may also explain these short-term events.…”

Kepler K2 and TESS Observations of Two Magnetic Cataclysmic Variables: The New Asynchronous Polar SDSS J084617.11+245344.1 and Paloma

“…The SAP light curve of Paloma shows a gentle parabolic curvature, while the PDCSAP light curve lacks any overall trend. To determine which light curve to use, we follow the general approach of Hill et al (2022) and compare both the SAP and PDCSAP light curves against simultaneous r-band photometry from the Zwicky Transient Facility (ZTF; Bellm et al 2019). We used linear regressions to compare the flux of the seven available ZTF observations with the simultaneous TESS observations and found that the PDCSAP light curve agreed well with the ZTF data, with a coefficient of determination of = r 0.54 , but there are no obvious indicators that that particular ZTF measurement is unreliable.…”