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Transiting Exoplanet Survey Satellite (TESS) photometry of the polars AM Herculis (AM Her) and AR Ursae Majoris (AR UMa) is presented, along with high-speed photometry. AM Her shows a variety of high states with frequent transitions between them. TESS photometry of AR UMa in the low state reveals no evidence of accretion, while the McDonald 2.1 m telescope caught AR UMa in its high accretion state. Roche-lobe overflow is shut off during low states of AR UMa, while accretion often still takes place during low states of AM Her. We derive inclinations of 50° and 70° for AM Her and AR UMa respectively. To model the high-state light curves of AM Her, we employ a self-organized map light-curve classification scheme to establish common accretion configurations. The cyclotron radiation properties then allow the production of emission region maps on the surface of the white dwarf. The accretion geometry of AM Her is most consistent with a multipolar field structure. The high-state photometry of AR UMa has stochastic accretion flaring, which we attribute to magnetically buffeted mass transfer through the inner Lagrangian point L1. To consider this possibility, we examine the magnetism of both stars and argue that the local magnetic field near L1 can initiate short-lived accretion events and affect transitions between high and low accretion states in both AM Her and AR UMa. In particular, AR UMa has the low state as its default, while AM Her and most other active polars are in the high state by default.

 

Cataclysmic variables (CVs) that have evolved past the period minimum during their lifetimes are predicted to be systems with a brown dwarf donor. While population synthesis models predict that around 40–70 per cent of the Galactic CVs are post-period minimum systems referred to as ‘period bouncers’, only a few dozen confirmed systems are known. We report the study and characterization of a new eclipsing CV, SRGeJ041130.3+685350 (SRGeJ0411), discovered from a joint SRG/eROSITA and ZTF programme. The optical spectrum of SRGeJ0411 shows prominent hydrogen and helium emission lines, typical for CVs. We obtained optical high-speed photometry to confirm the eclipse of SRGeJ0411 and determine the orbital period to be Porb ≈ 97.530 min. The spectral energy distribution suggests that the donor has an effective temperature of ≲ 1800 K. We constrain the donor mass with the period–density relationship for Roche lobe-filling stars and find that Mdonor ≲ 0.04 M⊙. The binary parameters are consistent with evolutionary models for post-period minimum CVs, suggesting that SRGeJ0411 is a new period bouncer. The optical emission lines of SRGeJ0411 are single-peaked despite the system being eclipsing, which is typically only seen due to stream-fed accretion in polars. X-ray spectroscopy hints that the white dwarf in SRGeJ0411 could be magnetic, but verifying the magnetic nature of SRGeJ0411 requires further investigation. The lack of optical outbursts has made SRGeJ0411 elusive in previous surveys, and joint X-ray and optical surveys highlight the potential for discovering similar systems in the near future.

 

White dwarfs that accrete from non-degenerate companions show anomalous carbon and nitrogen abundances in the photospheres of their stellar components have been postulated to be descendants of supersoft X-ray binaries. Measuring the carbon-to-nitrogen abundance ratio may provide constraints on their past evolution. We fit far-ultraviolet spectroscopy of the cataclysmic variable HS 0218 + 3229 taken with the Cosmic Origins Spectrograph using Markov chain Monte Carlo methods, and found the carbon-to-nitrogen ratio is about one tenth of the Solar value $(\rm{\log \mathrm{[C/N]}}=-0.56\pm 0.15)$. We also provide estimates of the silicon and aluminium abundances, and upper limits for iron and oxygen. Using the parameters we derived for HS 0218 + 3229 we reconstruct its past. We calculated a grid of mesa models and implemented Gaussian process fits in order to determine its most likely initial binary configuration. We found that an initial mass of the donor of $M_{\rm donor;i}=0.90-0.98,\rm{\mathrm{M}_{\odot }}$ and an initial orbital period of Porb; i = 2.88 d (Porb; i = 3.12–3.16 d) for an assumed initial white dwarf mass of $\rm{M_{\mathrm{WD}}}_\mathrm{;i}=0.83\, \rm{\mathrm{M}_{\odot }}$$(\rm{M_{\mathrm{WD}}}_{\rm ;i}=0.60\, \rm{\mathrm{M}_{\odot }})$ can replicate the measured parameters. The low mass ratio, $M_{\rm donor;i} / \rm{M_{\mathrm{WD}}}_{\rm ;i} =1.08-1.18\, (1.5-1.63)$, suggests that the system did not go through a phase of hydrogen-burning on the white dwarf’s surface. However, we can not exclude a phase of thermal time-scale mass transfer in the past. We predict that HS 0218 + 3229 will evolve below the ≃ 76.2 ± 1 min period minimum for normal cataclysmic variables.

 

The orbital-period (P_orb) gap in the population of cataclysmic variables (CVs) informs the theoretical narrative of CV evolution, yet a complete understanding of the driving angular momentum loss mechanisms above and below this gap remains elusive. Here we identify, for standard CVs, a new, apparently monotonic relationship between quiescent color (G_BP–G_RP), absolute magnitude (M_G), andP_orb(between 70 minutes and 8 hr) revealed in Gaia DR2 and EDR3. We show thatP_orbincreases in the color–absolute-magnitude diagram roughly orthogonally to the white dwarf and main sequences. We find the orbital-period–color–absolute-magnitude relationship to be stable across different CV subtypes: dwarf novae, intermediate polars, polars, and novalike systems. We place our findings in context with the known semiempirical donor sequence for CVs and find a dependence between color andM_Gfor a givenP_orbspecifically for dwarf novae and intermediate polars above the period gap. These relations have the potential to inform a more complete picture of CV evolution.

 

Abstract Magnetic cataclysmic variables (CVs) are luminous Galactic X-ray sources, which have been difficult to find in purely optical surveys due to their lack of outburst behavior. The eROSITA telescope on board the Spektr-RG mission is conducting an all-sky X-ray survey and recently released the public eROSITA Final Equatorial Depth Survey (eFEDS) catalog. We crossmatched the eFEDS catalog with photometry from the Zwicky Transient Facility and discovered two new magnetic CVs. We obtained high-cadence optical photometry and phase-resolved spectroscopy for each magnetic CV candidate and found them both to be polars. Among the newly discovered magnetic CVs is eFEDS J085037.2+044359/ZTFJ0850+0443, an eclipsing polar with orbital period P orb = 1.72 hr and WD mass M WD = 0.81 ± 0.08 M ⊙ . We suggest that eFEDS J085037.2+044359/ZTFJ0850+0443 is a low magnetic field strength polar, with B WD ≲ 10 MG. We also discovered a non-eclipsing polar, eFEDS J092614.1+010558/ZTFJ0926+0105, with orbital period P orb = 1.47 hr and magnetic field strength B WD = 36–42 MG. 

There have been relatively few published long-duration, uninterrupted light curves of magnetic cataclysmic variable stars in which the accreting white dwarf’s rotational frequency is slightly desynchronized from the binary orbital frequency. We report Kepler K2 and TESS observations of two such systems. The first, SDSS J084617.11+245344.1, was observed by the Kepler spacecraft for 80 days during Campaign 16 of the K2 mission, and we identify it as a new asynchronous polar with a likely 4.64 hr orbital period. This is significantly longer than any other asynchronous polar, as well as all but several synchronous polars. Its spin and orbital periods beat against each other to produce a conspicuous 6.77-day beat period, across which the system’s accretion geometry gradually changes. The second system in this study, Paloma, was observed by TESS for one sector and was already known to be asynchronous. Until now, there had been an ambiguity in its spin period, but the TESS power spectrum pinpoints a spin period of 2.27 hr. During the resulting 0.7-day spin–orbit beat period, the light curve phased on the spin modulation alternates between being single and double humped. We explore two possible explanations for this behavior: the accretion flow being diverted from one of the poles for part of the beat cycle, or an eclipse of the emitting region responsible for the second hump.

 

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 tog= 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.

 

Since its discovery in 1995, V2400 Ophiuchi (V2400 Oph) has stood apart from most known intermediate polar cataclysmic variables due to its proposed magnetic field strength (9–27 MG) and diskless accretion. To date, the exact accretion mechanism of the system is still unknown, and standard accretion models fail to accurately predict the peculiar behavior of its light curve. We present the K2 Campaign 11 light curve of V2400 Oph recording 74.19 days of photometric data cadenced at 1 minute. The light curve is dominated by aperiodic flickering and quasiperiodic oscillations, which make the beat and spin signals inconspicuous on short timescales. Notably, a log–log full power spectrum shows a break frequency at ∼10²cycles d⁻¹similar to some disk-fed systems. Through power-spectral analysis, the beat and spin periods are measured as 1003.4 ± 0.2 s and 927.7 ± 0.1 s, respectively. A power spectrum of the entire K2 observation demonstrates beat period dominance. However, time-resolved power spectra reveal a strong dependence between observation length and the dominant frequency of the light curve. For short observations (2–12 hr) the beat, spin, or first beat harmonic can be observed as the dominant periodic signal. Such incoherence and variability indicate a dynamical accretion system more complex than current intermediate polar theories can explain. We propose that a diamagnetic blob accretion model may serve as a plausible explanation for the accretion mechanism.

 

Magnetically gated accretion has emerged as a proposed mechanism for producing extremely short, repetitive bursts of accretion onto magnetized white dwarfs in intermediate polars (IPs), but this phenomenon has not been detected previously in a confirmed IP. We report the 27 day TESS light curve of V1025 Cen, an IP that shows a remarkable series of 12 bursts of accretion, each lasting for less than 6 hours. The extreme brevity of the bursts and their short recurrence times (∼1–3 days) are incompatible with the dwarf-nova instability, but they are natural consequences of the magnetic gating mechanism developed by Spruit and Taam to explain the Type II bursts of the accreting neutron star known as the Rapid Burster. In this model, the accretion flow piles up at the magnetospheric boundary and presses inward until it couples with the star’s magnetic field, producing an abrupt burst of accretion. After each burst, the reservoir of matter at the edge of the magnetosphere is replenished, leading to cyclical bursts of accretion. A pair of recent studies applied this instability to the suspected IPs MV Lyr and TW Pic, but the magnetic nature of these two systems has not been independently confirmed. In contrast, previous studies have unambiguously established the white dwarf in V1025 Cen to be significantly magnetized. The detection of magnetically gated bursts in a confirmed IP therefore validates the extension of the Spruit and Taam instability to magnetized white dwarfs.