Discovery of a Pulse-phase-transient Cyclotron Line in the X-Ray pulsar GRO J2058+42

We report the discovery of absorption features in the X-ray spectrum of the transient X-ray pulsar GRO J2058+42. The features are detected around ∼10, ∼20, and ∼30 keV in both NuSTAR observations carried out during the source type II outburst in spring 2019. The most intriguing property is that the deficit of photons around these energies is registered only in the narrow phase interval covering around 10% of the pulsar spin period. We interpret these absorption lines as a cyclotron resonant scattering line (fundamental) and two higher harmonics. The measured energy allow us to estimate the magnetic field strength of the neutron star as ∼1012 G.