Cyclotron lines: from magnetic field strength estimators to geometry tracers in neutron stars

TL;DR

Cyclotron lines in neutron stars serve as key tools for measuring magnetic fields, understanding accretion geometry, and studying emission properties, with recent focus on pulse phase spectroscopy and long-term line evolution.

Contribution

This review highlights the advancements in understanding cyclotron lines, emphasizing pulse phase spectroscopy and the physics of accretion in neutron stars.

Findings

Cyclotron line energies vary with luminosity and pulse phase.

Harmonic separations provide insights into magnetic field structure.

Long-term evolution of line energy reveals changes in magnetic field or accretion processes.

Abstract

With exactly forty years since the discovery of the first cyclotron line in Her X-1, there have been remarkable advancements in the field related to study of the physics of accreting neutron stars -- cyclotron lines have been a major torchbearer in this regard, from being the only direct estimator of the magnetic field strength, a tracer of accretion geometry and an indicator of the emission beam in these systems. The main flurry of activities have centred around studying the harmonic separations, luminosity dependence, pulse phase dependence and more recently the shapes of the line and the trend for long term evolution in the line energy. This article visits the important results related to cyclotron lines since its discovery and reviews their significance. An emphasis is laid on pulse phase resolved spectroscopy and the important clues a joint timing and spectral study in this context can provide, to build a complete picture for the physics of accretion and hence X-ray emission in accreting neutron stars.