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

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.
Has appeared in Journal of Astrophysics and Astronomy special issue on 'Physics of Neutron Stars and related objects', celebrating the 75th birth-year of G. Srinivasan