Evidence for strong cyclotron line emission in the hard X-ray spectrum of Hercules X-1.

Further results of the balloon observation of Hercules X-1 on May 3, 1976, are presented which confirm the previously reported existence of a strong line feature at about 58 keV in the pulsed (1.24 s) X-ray spectrum. The spectral excess in the line region over the extrapolated continuum is 5 to 6 sigmas. The best estimate of the line flux and line width are 0.003 photon/sq cm per sec and less than about 12 keV, respectively. The most likely interpretation of this line is electron cyclotron emission at the fundamental frequency from the hot polar plasma of the rotating neutron star. The corresponding magnetic-field strength is 5.3 trillion gauss. A search for the second-harmonic cyclotron emission line in that part of the data showing the highest signal-to-noise ratio reveals a 3.3-sigma spectral enhancement near the predicted energy (110 keV). Implications of the line width and the line intensity ratio for the physical conditions of the emitting plasma and the beaming geometry are discussed.