Detection of fundamental and first harmonic cyclotron line in X-ray pulsar Cep X-4

TL;DR

This study analyzes Suzaku data of the X-ray pulsar Cep X-4, revealing a fundamental cyclotron line at 28 keV and its first harmonic at 45 keV, with phase-dependent variations indicating complex magnetic field geometry.

Contribution

First detection of the first harmonic cyclotron line at 45 keV in Cep X-4 and analysis of its phase-dependent properties, providing insights into magnetic field structure.

Findings

Detection of a cyclotron line at ~28 keV

Identification of a first harmonic at ~45 keV

Significant phase variation of line parameters

Abstract

We report the broad-band spectral properties of the X-ray pulsar Cep X-4 by using a Suzaku observation in 2014 July. The 0.8-70 keV spectrum was found to be well described by three continuum models - Negative and Positive power-law with Exponential cutoff (NPEX), high energy cutoff power-law and CompTT models. Additional components such as a cyclotron line at $\sim$28 keV and two Gaussian components for iron lines at 6.4 and 6.9 keV were required in the spectral fitting. Apart from these, an additional absorption feature at $\sim$45 keV was clearly detected in residuals obtained from the spectral fitting. This additional feature at $\sim$45 keV was clearly seen in phase-resolved spectra of the pulsar. We identified this feature as the first harmonic of the fundamental cyclotron line at $\sim$28 keV. The ratio between the first harmonic and fundamental line energies (1.7) was found to be in disagreement with the conventional factor of 2, indicating that the heights of line forming regions are different or viewed at larger angles. The phase-resolved spectroscopy of the fundamental and first harmonic cyclotron lines shows significant pulse-phase variation of the line parameters. This can be interpreted as the effect of viewing angle or the role of complicated magnetic field of the pulsar.