Distorted cyclotron line profile in Cep X-4 as observed by NuSTAR

TL;DR

This paper analyzes NuSTAR and Swift observations of Cep X-4, revealing a distorted cyclotron line profile with an additional absorption feature, and discusses its luminosity dependence and similarities to Vela X-1.

Contribution

It introduces a detailed spectral analysis of Cep X-4, highlighting the complex shape of the cyclotron line and its asymmetry, which previous models could not fully explain.

Findings

Detection of a distorted cyclotron line profile around 30 keV.

Identification of an additional absorption feature at 19 keV.

Small positive correlation between line energy and luminosity.

Abstract

We present spectral analysis of NuSTAR and Swift observations of Cep X-4 during its outburst in 2014. We observed the source once during the peak of the outburst and once during the decay, finding good agreement in the spectral shape between the observations. We describe the continuum using a powerlaw with a Fermi-Dirac cutoff at high energies. Cep X-4 has a very strong cyclotron resonant scattering feature (CRSF) around 30 keV. A simple absorption-like line with a Gaussian optical depth or a pseudo-Lorentzian profile both fail to describe the shape of the CRSF accurately, leaving significant deviations at the red side of the line. We characterize this asymmetry with a second absorption feature around 19 keV. The line energy of the CRSF, which is not influenced by the addition of this feature, shows a small but significant positive luminosity dependence. With luminosities between (1-6)e36 erg/s, Cep X-4 is below the theoretical limit where such a correlation is expected. This behavior is similar to Vela X-1 and we discuss parallels between the two systems.