The various accretion modes of AM Herculis: Clues from multi-wavelength observations in high accretion states

Schwope, A. D.; Wörpel, H.; Traulsen, I.; Sablowski, D. P.

doi:10.1051/0004-6361/202037714

Cited by 16 publications

(15 citation statements)

References 59 publications

(79 reference statements)

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“…The hypothesis of magnetically-driven accretion is observationally confirmed for the highly-magnetic CVs (polars) such as AM Herculis (Gänsicke et al 2001;Schwope et al 2020). But although they provide the closest, well-studied analogy to debris-accreting white dwarfs, it is important to understand that the range of accretion rates of 10 7 − 10 11 g s −1 estimated for metalpolluted white dwarfs (Fig.…”

Section: Analogy To Accretion On Cvsmentioning

confidence: 96%

Horizontal spreading of planetary debris accreted by white dwarfs

Cunningham

Tremblay

Bauer

et al. 2021

Monthly Notices of the Royal Astronomical Society

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White dwarfs with metal-polluted atmospheres have been studied widely in the context of the accretion of rocky debris from evolved planetary systems. One open question is the geometry of accretion and how material arrives and mixes in the white dwarf surface layers. Using the 3D radiation-hydrodynamics code CO5BOLD, we present the first transport coefficients in degenerate star atmospheres which describe the advection-diffusion of a passive scalar across the surface-plane. We couple newly derived horizontal diffusion coefficients with previously published vertical diffusion coefficients to provide theoretical constraints on surface spreading of metals in white dwarfs. Our grid of 3D simulations probes the vast majority of the parameter space of convective white dwarfs, with pure-hydrogen atmospheres in the effective temperature range 6000–18 000 K and pure-helium atmospheres in the range 12 000–34 000 K. Our results suggest that warm hydrogen-rich atmospheres (DA; ≳ 13 000 K) and helium-rich atmospheres (DB, DBA; ≳ 30 000 K) are unable to efficiently spread the accreted metals across their surface, regardless of the time dependence of accretion. This result may be at odds with the current non-detection of surface abundance variations at white dwarfs with debris discs. For cooler hydrogen- and helium-rich atmospheres, we predict a largely homogeneous distribution of metals across the surface within a vertical diffusion timescale. This is typically less than 0.1 per cent of disc lifetime estimates, a quantity which is revisited in this paper using the overshoot results. These results have relevance for studies of the bulk composition of evolved planetary systems and models of accretion disc physics.

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Section: Analogy To Accretion On Cvsmentioning

confidence: 96%

Horizontal spreading of planetary debris accreted by white dwarfs

Cunningham

Tremblay

Bauer

et al. 2021

Monthly Notices of the Royal Astronomical Society

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“…However, taking at face value, the implied sizes of the emission region are larger than those found e.g. in the prototypical object AM Herculis (about 100 km, Schwope et al 2020) but of similar size as in the textbook object EF Eri (< 570 km Beuermann et al 1987) and thus approximately as expected given the large uncertainties of the derived values and on the heating processes in this object. For a discussion of sizes of emission regions, see e.g.…”

Section: X-ray Spectroscopymentioning

confidence: 49%

“…In other polars such a dip occurs between binary phases ∼0.93 and ∼0.8 (V808 Aur, Worpel & Schwope 2015; AM Her, Schwope et al 2020). If this applies to V496 UMa, we may expect the orbital phase of hump1 between phases 0.0 and 0.12.…”

Section: Accretion Geometrymentioning

confidence: 97%

XMM-Newton and TESS observations of the highly variable polar V496 UMa

Ok,

Schwope

2022

Preprint

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Aims. We aim to study the temporal and spectral behaviour of V496 UMa from the optical to the X-ray regimes. Methods. We used archival XMM-Newton and TESS observations obtained in 2017 and 2019, respectively, to perform a spectral and timing analysis of the highly variable polar. Results. The light curves of both satellites, TESS and XMM-Newton, reveal a double-humped pattern modulated with the periodicity of 91.058467 ± 0.00001 minutes. V496 UMa displays a two-pole accretion geometry in the high accretion state. X-ray spectra from both regions are composed of thermal plasma radiation and soft blackbody components with almost identical temperatures and total accretion rate of Ṁ = 1.4(8) × 10 −11 M yr −1 . The X-ray centers of the humps show longitudinal shifts of −18 • and 4 • and −172 • and −186 • size around photometric phase zero, for the main hump and second hump, respectively. The long-term ZTF light curves reveal high and low accretions states. Low-state ZTF and SDSS photometric data are consistent with an 0.8 M white dwarf at 10000 K and a main-sequence donor star with a spectral type of M5.0 at a Gaia-determined distance of 758 pc. Conclusions. V496 UMa is a very bright polar in X-rays when it is in the high state. Due to its unusual geometric structure, mass accretion onto the second accretion pole is interrupted occasionally. This discontinuous behavior does not follow a certain pattern in time and is observed so far only in the high state. The X-ray light curves display clear evidence for an accretion stream at the photometric phase of φ =0.81, which does not show up in optical light curves. An accurate period was derived using the combined TESS and XMM-Newton data, which differs by 3.8 σ from published results.

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“…AM Her has been observed in both one-pole and two-pole configurations, but the timescale for switching between configurations is months-years. For short epochs of observations (∼ months), the accretion geometry seems stable (see Schwope et al 2020 for a thorough review on the variability seen in AM Her).…”

Section: Introductionmentioning

confidence: 99%

“…Such a model has been applied to explain the different accretion regimes within AM Her, and predicts significantly different X-ray spectra from the primary and secondary poles. (Hameury & King 1988;Schwope et al 2020).…”

Section: Introductionmentioning

confidence: 99%

Transient two pole accretion in the polar V496 UMa

Kennedy,

Littlefield,

Garnavich

2022

Preprint

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We report XMM-Newton and TESS observations of V496 UMa, an AM Herculis-type cataclysmic variable. The XMM-Newton observation reveals that at times, two poles on the white dwarf accrete simultaneously, but accretion onto the secondary magnetic pole is erratic and can nearly cease in less than one binary orbit (1.5 h). Modelling of the X-ray spectrum during the primary maximum reveals no change in the accretion structures onto the primary pole when accretion onto the secondary pole is disrupted, suggesting that the disruption of accretion onto the secondary pole may be caused by mass-transfer variations from the donor star. The TESS observation, which spanned eight weeks at a two-minute cadence, shows a stable, double-humped orbital modulation due to cyclotron emission from the post-shock region, while the observed times of maximum light show a slow systematic drift that does not correlate with the system's overall brightness.

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The various accretion modes of AM Herculis: Clues from multi-wavelength observations in high accretion states

Cited by 16 publications

References 59 publications

Horizontal spreading of planetary debris accreted by white dwarfs

Horizontal spreading of planetary debris accreted by white dwarfs

XMM-Newton and TESS observations of the highly variable polar V496 UMa

Transient two pole accretion in the polar V496 UMa

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