Accretion funnels in AM Herculis systems -- I. Model characteristics

Abstract

We present calculations of the thermal structure of accretion funnels heated by hard X‐ray emission from the accretion shock, soft X‐ray reprocessed radiation from the white dwarf surface and magnetic heating in the matter–field interaction region close to the orbital plane. The calculations determine self‐consistently the thermal structure of the funnel, allowing for radiative transfer, electron scattering and the trapping of radiation within the funnel, and represent a substantial improvement on previous purely kinematical models. We show that, while models that allow only for X‐ray heating can explain the observed intensities of the Balmer lines, they cannot, at the same time, explain the intensities of the He i and He ii lines in the optical spectra. These lines appear to be formed mainly in the magnetically heated transition region near the orbital plane, with this region playing a role similar to the hotspot in accretion discs. We show that, with the inclusion of this region, models can be constructed that are in close agreement with the optical line and continuum emission observed in AM Herculis systems: that is, they exhibit a flat or inverted Balmer decrement, He i lines, a strong He iiλ4686 line and complex emission‐line profiles which vary dramatically in velocity and shape over the orbital period of the white dwarf. We also show that the continuum emission from the accretion funnel provides an important source of unpolarized background radiation, which reduces the degree of polarization of the cyclotron radiation from the accretion shocks, and produces the polarization standstills that are a well‐known characteristic of these systems.

MASTER OT J132104.04+560957.8: A Polar with Absorption–Emission Line Reversals

“…One possible explanation for these observations is that the entire curtain is significantly hotter than predicted by Ferrario & Wehrse (1999). Unless irradiation-induced heating is stronger than predicted, this conclusion implies the existence of an additional mechanism that heats the entire curtain.…”

“…On one hand, the fact that H, He I, and He II all undergo the same blueward motion during the line inversion suggests that they share a common line-formation region within the curtain. Although the model of Ferrario & Wehrse (1999) predicts that the threading region should be the primary line-formation region for He II, the same argument regarding the He I absorption applies to He II; the threading region does not cross our line of sight to J1321's accretion region and therefore cannot be the source of the He II absorption. Consequently, the He II-absorbing material must reside within the curtain.…”

“…Emission-absorption reversals can be used to assess models of the line-producing regions of the accretion curtain. Ferrario & Wehrse (1999) modeled the H, He I, and He II emission of accretion funnels and found that a combination of irradiation and magnetic heating was necessary to reproduce the observed line strengths in polars. A profile of He II λ 468.6 nm, shown as the superposition of two Gaussians representing emission and absorption components.…”

High-speed, energy-resolved STJ observations of the AM Her system V2301 Oph

“…The material then travels along the field lines, until it forms a column of material settling onto the surface of the white dwarf via a shock (e.g. Ferrario & Wehrse 1999). In some systems (e.g.…”

IGR J19552+0044: A new asynchronous short period polar

“…The mass transfer and accretion in polars including APs takes place under strong influence of the magnetosphere of the WD and is very different from the remaining CVs (see reviews by Cropper 1990;Ferrario & Wehrse 1999). Observationally, three distinct components of emission lines were identified in polars .…”