Radiation-dominated polar emitting region of an accreting X-ray pulsar -- I. Polarization- and spectrum-dependent structure, and the emergent continuum

TL;DR

This work develops a self-consistent, polarization-aware model of the radiation-dominated accretion column at a neutron-star magnetic pole, solving coupled hydrodynamics and frequency-dependent radiative transfer for two polarization modes in a dipole-field geometry. It demonstrates that frequency- and polarization-dependent magnetic cross sections, along with first- and second-order bulk Comptonization and induced scattering, shape the column structure, electron temperature, and the emergent continuum and polarization across a range of accretion rates. The results reveal a nonlinear height evolution of the column, a high-energy tailenhanced by bulk effects, and polarization fractions that rise with accretion rate, with the extraordinary mode typically dominating near the spectral peak. The study identifies key physical channels governing X-ray continua in luminous pulsars and outlines clear avenues for improved fits to observations through inclusion of cyclotron resonances, angular radiative transfer, and more complex magnetic-field geometries, informing interpretation of CRSFs and high-energy spectral shapes.

Abstract

The radiation-dominated polar emitting region of an accreting X-ray pulsar is simulated numerically in the framework of a three-dimensional (geometrically two-dimensional) model. The radiative transfer within the emitting region and the structure of the latter are calculated with the use of the self-consistent algorithm developed earlier. The magnetic scattering cross sections dependent on the photon energy and polarization have been incorporated. Second-order bulk Comptonization over entire emitting region, induced Compton scattering, the switching of the polarization modes, free-free processes, the cyclotron emission because of electron-proton collisions, and a realistic shape of the accretion channel have been taken into account. The case of a dipole magnetic field is considered. It is shown that the induced Compton effect can play a notable role in establishing the electron temperature in the post-shock zone. Within the model shock wave, a higher electron temperature is achieved than in the post-shock zone by means of the bulk-heating mechanism. The photons gaining the energy in the shock wave and above it due to bulk motion effects and the thermal Doppler effect are responsible for the formation of high-energy regions in the emergent continuum of the polarization modes.

Figures (9)

• Figure 1: Column grown in the filled accretion funnel, calculations for $\dot M_{17}=1$: (top panels) the structure, (middle) the spectrum of the radiation in the number of points within the funnel, (left) $\theta=0$ and (right) $\theta=0.07$, (bottom left) the spectral luminosity of the radiation from the side boundary of the funnel, and (bottom right) the polarization fraction for the emergent radiation.
• Figure 2: Same as Fig. \ref{['fig:str1']}, but for $\dot M_{17}=3$. In the right panel, the spectral radiation energy density is shown for $\theta=0.087$.
• Figure 3: Same as Fig. \ref{['fig:str1']}, but for $\dot M_{17}=5$. In the right panel, the spectral radiation energy density is shown for $\theta=0.093$.
• Figure 4: Same as Fig. \ref{['fig:str1']}, but for the hollow funnel. The spectral radiation energy density is shown for (left) $\theta\approx (\theta_1+\theta_2)/2$ and (right) $\theta\approx 0.0875$.
• Figure 5: Same as Fig. \ref{['fig:str1']}, but for $v(r_{\rm up})=1.3\times 10^{10}$ cm s$^{-1}$ (panels for the local spectra are not shown).