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We present spectral analysis of NuSTAR and Swift observations of Cep X-4during its outburst in 2014. We observed the source once during the peak of theoutburst and once during the decay, finding good agreement in the spectralshape between the observations. We describe the continuum using a powerlaw witha Fermi-Dirac cutoff at high energies. Cep X-4 has a very strong cyclotronresonant scattering feature (CRSF) around 30 keV. A simple absorption-like linewith a Gaussian optical depth or a pseudo-Lorentzian profile both fail todescribe the shape of the CRSF accurately, leaving significant deviations atthe red side of the line. We characterize this asymmetry with a secondabsorption feature around 19 keV. The line energy of the CRSF, which is notinfluenced by the addition of this feature, shows a small but significantpositive luminosity dependence. With luminosities between (1-6)e36 erg/s, CepX-4 is below the theoretical limit where such a correlation is expected. Thisbehavior is similar to Vela X-1 and we discuss parallels between the twosystems.

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