White Dwarf Masses in Intermediate Polars Determined from X-ray Spectra.

Analyses of hard X-ray spectra from 47 intermediate polars reveal an average white dwarf mass of 0.82 solar masses, consistent with cataclysmic variable estimations. This validates current models assuming magnetospheric radii approximate corotation radii and small accretion column heights, though low-luminosity systems require individual modelling.

Determining the mass of white dwarf stars within binary systems known as intermediate polars presents a significant challenge for astronomers. These compact stellar remnants accrete material from a companion star, generating intense X-ray emission that encodes information about the white dwarf’s fundamental properties. A new analysis, detailed in the article ‘X-ray spectroscopy method of white dwarf mass determination in intermediate polars. External systematic uncertainties’, focuses on refining the precision of mass calculations derived from X-ray spectra, carefully assessing the impact of various systematic errors inherent in the modelling process. Researchers V. F. Suleimanov, L. Ducci, V. Doroshenko and K. Werner, all affiliated with the Institut für Astronomie und Astrophysik and the Kepler Center for Astro and Particle Physics at the Universität Tübingen, present a comprehensive investigation into these uncertainties, including the geometry of the accretion flow and the potential influence of heated stellar surfaces. Their work utilises a new model grid of X-ray spectra, applied to data from the Swift/BAT telescope, to analyse a sample of 47 intermediate polars and establish a more robust understanding of white dwarf masses within these systems.

White Dwarf Masses in Intermediate Polars: A Spectroscopic Analysis

Researchers have refined the determination of white dwarf (WD) masses within intermediate polar (IP) systems using hard X-ray spectroscopy, with a detailed assessment of systematic uncertainties. The study focuses on the influence of magnetospheric radii, accretion column geometry, and the impact of accretion-heated envelopes on WD radii, improving the accuracy of existing models. A new grid of hard X-ray spectra, incorporating a revised mass-radius relation for WDs, was generated and applied to analyse Swift/BAT spectra from a sample of 47 IPs, contributing to a more comprehensive understanding of these binary systems.

The analysis reveals an average WD mass of 0.82 solar masses within the sample, consistent with mass determinations from other cataclysmic variables (CVs) and validating current spectroscopic models. Crucially, researchers demonstrate that accretion-heated envelopes significantly alter the standard mass-radius relation applicable to cold WDs, impacting the precision of mass estimations.

Accretion-heated envelopes, modelled with surface temperatures of 30 kK, demonstrably affect WD radii and improve the accuracy of mass estimations. The study highlights the limitations of applying a universal spectral grid to all IPs, acknowledging the inherent diversity within this class of binary systems and advocating for tailored approaches.

This work establishes a methodology for determining WD masses within IP CVs through hard X-ray spectral analysis, meticulously considering systematic uncertainties. The investigation actively addresses the influence of accretion-heated envelopes on WD radii, recognising that these envelopes modify the standard mass-radius relationship for cold white dwarfs. Specifically, hydrogen envelope models with a surface temperature of 30 kK approximate the effects of accretion heating, enhancing the accuracy of mass estimations and providing a more realistic representation of the physical conditions within these systems.

The study presents a new grid of hard X-ray spectra for high-luminosity IPs, facilitating the analysis of Swift/BAT spectra from a sample of 47 systems and providing a robust dataset for comparison. This consistency validates the underlying assumptions of the hard X-ray spectral modelling, namely that magnetospheric radii approximate corotation radii and that accretion column heights remain relatively small in high-luminosity systems, solidifying the reliability of the approach.

Researchers identify that low-luminosity IPs likely possess comparatively tall accretion columns, necessitating individual modelling to achieve accurate results and refine our understanding of their unique characteristics. The height of the accretion column influences the observed X-ray spectrum, and a uniform approach fails to capture the nuances of these systems. Consequently, the study advocates for individual accretion column modelling for low-luminosity IPs to achieve accurate mass determinations, acknowledging the varying geometries and physical conditions within the population of intermediate polars.

The study confirms the effectiveness of hard X-ray spectroscopy as a method for determining WD masses in IPs, while simultaneously identifying crucial considerations for refining the technique and addressing the diversity within this class of binary systems. The findings underscore the importance of accounting for accretion-heated envelopes and employing tailored models for low-luminosity IPs to ensure robust mass determinations, contributing to a more complete picture of these fascinating binary systems.