Cyclotron lines in the Her X-1 spectrum: structure and higher harmonics

Following the suggestion by Truemper that the recently observed hard X-ray peak in the Her X-1 spectrum represents cyclotron emission in a magnetic field approximately 5 1012 Gauss, we examine the observability of additional spectral features as predicted by the quantum theory of cyclotron radiation. The relative intensities of higher harmonics are expected to be strongly suppressed due to a quantum mechanical selection rule favoring de-excitation via single-step transitions. However, if the Landau levels n >= 2 are indeed populated, our calculation also shows that at 5 1012 Gauss the various Δn = 1 de-excitations radiate at slightly different frequencies, resulting in a splitting of the first harmonic by about 10%. Depending on strongly angle-dependent Doppler broadening effects this level splitting may be observable, and may provide information on the physics of the emission layer. (orig.)