Modeling Dipolar Post-Shock Accretion Columns for Various Specific Accretion Rate Intermediate Polars

TL;DR

This paper models the post-shock accretion columns in intermediate polars, considering various accretion rates, geometries, and compositions, to understand their structure and spectra.

Contribution

It introduces a detailed model of the PSAC incorporating dipolar geometry, variable accretion rates, and metal abundances, improving understanding of spectral differences in intermediate polars.

Findings

Dipolar geometry reduces density and temperature compared to cylindrical models.

Spectra soften more rapidly with decreasing accretion rate in dipolar models.

Different geometries lead to different white dwarf mass estimates from spectral fitting.

Abstract

We model the post-shock accretion column (PSAC) for intermediate polars (IPs), with parameterizing specific accretion rate between 0.0001 and 100 g cm-2 s-1 and metal abundance between 0.1 and 2 times of solar abundance, and taking into account the gravitational potential and non-equipartition between ions, electrons and ionization degree. We assume the cylinder and dipole as geometry of the PSAC. The PSAC becomes higher against the white dwarf (WD) radius for lower specific accretion rate and more massive WD, and may be comparable to the WD radius. The consideration of the dipolar geometry significantly reduces the density and temperature over the whole PSAC comparing with the cylindrical case when the specific accretion rate is lower than a threshold which the PSAC height reachs 0.2 RWD with and is decreased by the more massive white dwarf. We calculate the spectra of the cylindrical and dipolar PSACs with the wide range of the specific accretion rate. Although the spectra soften as the specific accretion rate decreases for the both geometrical assumptions under the specific accretion rate threshold, the softening is more speedy for the dipolar PSAC. The fact means that the both geometrical assumptions lead the different WD masses for each other when their spectra are applied to the IPs hosting the low accretion or a massive WD. Although the ionization non-equilibrium are also involved for the spectral calculation, the effects are trivial because the radiation from ionization non-equilibrium plasma is a few percent of the whole at most.