A new soft X-ray spectral model for polars with an application to AM Herculis

We present a simple heuristic model for the time-averaged soft X-ray temperature distribution in the accretion spot on the white dwarf in polars. The model is based on the analysis of the Chandra LETG spectrum of the prototype polar AM Her and involves an exponential distribution of the emitting area vs. blackbody temperature a(T) = a0 exp(-T/T0). With one free parameter besides the normalization, it is mathematically as simple as the single blackbody, but is physically more plausible and fits the soft X-ray and far-ultraviolet spectral fluxes much better. The model yields more reliable values of the wavelength-integrated flux of the soft X-ray component and the implied accretion rate than reported previously.

💡 Research Summary

The paper addresses a long‑standing problem in the modeling of soft X‑ray emission from polars, using the prototype system AM Herculis as a test case. Historically, the soft X‑ray component has been approximated by a single‑temperature blackbody, a simplification that reproduces the overall shape of the Chandra Low‑Energy Transmission Grating (LETG) spectrum but fails to capture the physical reality of a temperature gradient across the accretion spot and leads to inconsistencies with far‑ultraviolet (FUV) observations.

To overcome these shortcomings, the authors propose a heuristic, yet physically motivated, model in which the emitting area as a function of temperature follows an exponential law:
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