Cyclotron absorption lines in the era of Suzaku and NuSTAR

Abstract

Cyclotron resonance scattering features or cyclotron absorption lines are unique features observed in the hard X-ray spectra of accretion powered X-ray pulsars with magnetic field of the order of 10$^{12}$ G. Detection of these features enables us for the direct estimation of strength of the magnetic field close to the neutron star surface. Corresponding to magnetic field of $\sim$10$^{12}$ G, the fundamental cyclotron lines are expected in 10-100 keV energy range with harmonics expected at multiples of fundamental line energy. However, we detected first harmonic of cyclotron line at less than twice of the fundamental line energy ($\sim$1.7 times the fundamental line energy) in Be/X-ray binary pulsar Cep X-4. With the broadband spectral capability of $Suzaku$ and $NuSTAR$ observatories, we have investigated cyclotron resonance scattering features in several X-ray pulsars to understand the shape of the lines, width, magnetic field mapping, anharmonicity in the line energies and luminosity-dependent properties of cyclotron lines. The results obtained from these works and new detection of cyclotron line in unknown/poorly studied sources are presented in this paper.