Opacities and spectra of hydrogen atmospheres of moderately magnetized neutron stars

TL;DR

This paper extends models of hydrogen atmospheres of neutron stars to include moderately strong magnetic fields ($10^{11}$ G to $10^{12}$ G), providing new equations of state and opacities relevant for observed CCOs.

Contribution

It develops and tabulates equations of state and radiative opacities for partially ionized hydrogen in magnetic fields $10^{10}$ G to $10^{12}$ G, filling a gap in existing models.

Findings

New tables of thermodynamic functions and opacities for specified magnetic fields and temperatures.

Atmosphere models confirm the importance of partial ionization effects in certain magnetic field and temperature ranges.

Results applicable to interpreting spectra of neutron stars with moderate magnetic fields.

Abstract

There is observational evidence that central compact objects (CCOs) in supernova remnants have moderately strong magnetic fields $B\sim10^{11}$ G. Meanwhile, available models of partially ionized hydrogen atmospheres of neutron stars with strong magnetic fields are restricted to $B\gtrsim10^{12}$ G. We extend the equation of state and radiative opacities, presented in previous papers for $10^{12}\mbox{ G}\lesssim B \lesssim 10^{15}$ G, to weaker fields. An equation of state and radiative opacities for a partially ionized hydrogen plasma are obtained at magnetic fields $B$, temperatures $T$, and densities $\rho$ typical for atmospheres of CCOs and other isolated neutron stars with moderately strong magnetic fields. The first- and second-order thermodynamic functions, monochromatic radiative opacities, and Rosseland mean opacities are calculated and tabulated, taking account of partial ionization, for $3\times10^{10}\mbox{ G}\lesssim B\lesssim 10^{12}$ G, $10^5$ K $\lesssim T\lesssim 10^7$ K, and a wide range of densities. Atmosphere models and spectra are calculated to verify the applicability of the results and to determine the range of magnetic fields and effective temperatures where the incomplete ionization of the hydrogen plasma is important.