Cyclotron harmonics in opacities of isolated neutron star atmospheres

TL;DR

This paper reviews the physics of cyclotron quantum harmonics in free-free photoabsorption in neutron star atmospheres, explaining their potential role in observed X-ray spectral features and why higher harmonics are unobservable.

Contribution

It provides a comprehensive review of cyclotron quantum harmonics in neutron star atmospheres and clarifies why ion cyclotron harmonics beyond the fundamental are not detectable.

Findings

Cyclotron harmonics can cause absorption features in neutron star spectra.

Ion cyclotron harmonics beyond the fundamental are not observable.

Different approximations for calculating these harmonics are discussed.

Abstract

Some of X-ray dim isolated neutron stars (XDINS) and central compact objects in supernova remnants (CCO) show absorption features in their thermal soft X-ray spectra. It has been hypothesized that these features could be due to the periodic peaks in free-free absorption opacities, caused by either Landau quantization of electron motion in magnetic fields B<10^{11} G or analogous quantization of ion motion in magnetic fields B>10^{13} G. Here, I review the physics behind cyclotron quantum harmonics in free-free photoabsorption, discuss different approximations for their calculation, and explain why the ion cyclotron harmonics (beyond the fundamental) cannot be observed.