A Magnetic Valve at L1 Revealed in TESS Photometry of the Asynchronous Polar BY Cam

Abstract

We present TESS photometry of the asynchronous polar BY Cam, which undergoes a beat cycle between the 199.384 min white dwarf (WD) spin period and the 201.244 min orbital period. This results in changes in the flow of matter onto the WD. The TESS light curve covers 92% of the beat cycle once and 71% of the beat cycle twice. The strongest photometric signal, at 197.560 min, is ascribed to a side-band period. During times of light-curve stability, the photometry modulates at the spin frequency, supporting our WD spin-period identification. Both one-pole and two-pole accretion configurations repeat from one beat cycle to the next with clear and repeatable beat-phase-dependent intensity variations. To explain these, we propose the operation of a magnetic valve at L1. The magnetic valve modulates the mass-transfer rate, as evidenced by a factor of 5 variation in orbital-averaged intensity, over the course of the beat cycle in a repeatable manner. The accretion stream threading distance from the WD is also modulated at the beat period, because of the variation of the WD magnetic field with respect to the stream and because of changes in the mass transfer rate due to the operation of the magnetic valve. Changes in the threading distance result in significant shifts in the position of accreting spots around the beat cycle. As a consequence, only the faintest photometric minima allow for an accurate ephemeris determination. Three regions on the WD appear to receive most of the accretion flow, suggestive of a complex WD magnetic field.

SDSS J134441.83+204408.3: A Highly Asynchronous Short-period Magnetic Cataclysmic Variable with a 56 MG Field Strength

“…Although APs once had very distinct P spin /P orb ratios compared to IPs, the unusual systems Paloma (P spin /P orb = 0.87; Schwarz et al 2007;Joshi et al 2016;Littlefield et al 2022) and Swift J0503.7-2819 (P spin /P orb = 0.8; Halpern 2022;Rawat et al 2022) have blurred the once-clear line of division between the APs and IPs. 10 Throughout this paper, we use the neutral term "asynchronous mCV" to refer to systems in which it is unknown whether the observed asynchronous rotation is an equilibrium condition.…”

“…Because this period can be days, weeks, or even months long, it is often infeasible to observe from the ground. The Kepler spacecraft and the Transiting Exoplanet Survey Satellite (TESS) have observed several APs, resulting in a flurry of publications about the APs: CD Ind (Hakala et al 2019;Littlefield et al 2019;Mason et al 2020), V1500 Cyg (Wang et al 2021), BY Cam (Mason et al 2022), and SDSS J0846 (Littlefield et al 2023), as well as Paloma (Littlefield et al 2023) and Swift J0503.7-2819 (Halpern 2022;Rawat et al 2022).…”

TESS and ground based photometry of magnetic CVs

“…An Interactive Magnetic Valve Model for mass transfer in polars, capable of modulating mass transfer over the beat cycle in the typical AP, BY Cam, was proposed [11]. Essentially the same model was applied to synchronous polars to explain their high and low accretion states [12] and to the low-field polar AM Her and the canonical high-field polar AR UMa in a joint study of TESS observations [13].…”

“…TESS and ground-based photometry of magnetic CVs Paul A. Mason enhance the plasma flow from the inner Lagrangian point on the donor [11,12,13]. A high-speed photometric flickering analysis of TESS observations of polars [16] and magnetic gating reported in the IP, TX Col [17], are likely both indicative of magnetically influenced accretion flows as is the presence of occasionally accreting polars in the 2-3 hr CV period gap [18].…”