We present a spectral analysis of three simultaneous NuSTAR and Swift /XRT observations of the transient Be-neutron star binary KS 1947+300 taken during its outburst in 2013/2014 .
These broad-band observations were supported by Swift /XRT monitoring snap-shots every 3 days , which we use to study the evolution of the spectrum over the outburst .
We find strong changes of the power-law photon index , which shows a weak trend of softening with increasing X-ray flux .
The neutron star shows very strong pulsations with a period of P \approx 18.8 s. The 0.8–79 keV broad-band spectrum can be described by a power-law with an exponential cutoff and a black-body component at low energies .
During the second observation we detect a cyclotron resonant scattering feature at 12.5 keV , which is absent in the phase-averaged spectra of observations 1 and 3 .
Pulse phase-resolved spectroscopy reveals that the strength of the feature changes strongly with pulse phase and is most prominent during the broad minimum of the pulse profile .
At the same phases the line also becomes visible in the first and third observation at the same energy .
This discovery implies that KS 1947+300 has a magnetic field strength of B \approx 1.1 \times 10 ^ { 12 } ( 1 + z ) G , which is at the lower end of known cyclotron line sources .