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We present a spectral analysis of three simultaneous NuSTAR and Swift/XRTobservations of the transient Be-neutron star binary KS 1947+300 taken duringits outburst in 2013/2014. These broad-band observations were supported bySwift/XRT monitoring snap-shots every 3 days, which we use to study theevolution of the spectrum over the outburst. We find strong changes of thepower-law photon index, which shows a weak trend of softening with increasingX-ray flux. The neutron star shows very strong pulsations with a period of P ~18.8 s. The 0.8-79 keV broad-band spectrum can be described by a power-law withan exponential cutoff and a black-body component at low energies. During thesecond observation we detect a cyclotron resonant scattering feature at 12.5keV, which is absent in the phase-averaged spectra of observations 1 and 3.Pulse phase-resolved spectroscopy reveals that the strength of the featurechanges strongly with pulse phase and is most prominent during the broadminimum of the pulse profile. At the same phases the line also becomes visiblein the first and third observation at the same energy. This discovery impliesthat KS 1947+300 has a magnetic field strength of B ~ 1.1e12 (1+z)G, which isat the lower end of known cyclotron line sources.

NuSTAR Discovery of a cyclotron line in KS 1947+300