Changes in the Nature of the Spectral Continuum and Stability of the Cyclotron Line in the X-ray Pulsar GRO J2058+42

We present the results of our study of the transient X-ray pulsar GRO J2058+42 in a wide energy range in a state with a luminosity $L_x∼eq 2.5× 10^{36}$ erg s${}^{-1}$. We have found that the pulse profile of the source, along with the pulse fraction, changed significantly in comparison with the previous NuSTAR observations performed when the pulsar was brighter approximately by a factor of 10. The position of the cyclotron line at ${∼}10$ keV in a narrow phase interval is consistent with the observations in the high state. Spectral analysis has shown that at high luminosities $L_x∼eq(2.7-3.2)× 10^{37}$ erg s${}^{-1}$ the spectrum has a shape typical of accreting pulsars, whereas a two-component model should be used to describe the spectrum when the luminosity drops approximately by an order of magnitude, to $2.5× 10^{36}$ erg s${}^{-1}$. This behavior fits into the model in which the low-energy part of the spectrum is formed in a hot spot, while the high-energy one is formed as a result of resonant Compton scattering by the infalling matter in the accretion channel above the neutron star surface.