Cyclotron radiation by anisotropic relativistic electrons in X-ray pulsars

We calculate the cyclotron-radiation spectrum for relativistic electrons wi th an anisotropic distribution in which the velocity spread across the magn etic held is much less than the speed of light. If the cyclotron feature in the spectrum of Her X-l is assumed to be due to such electrons, the implie d magnetic field is (4-6) x 10(10) G, in good agreement with other observat ional and theoretical estimates. The observed temporal variation of the ene rgy of the cyclotron lines in the spectra of some pulsars can be explained by variability of the mean longitudinal velocity of the electrons, which de creases with increasing luminosity due to increase of the radiative deceler ation of the flow.