SPECTRAL BEHAVIOR OF HERCULES-X-1 - ITS LONG-TERM VARIABILITY AND PULSE PHASE SPECTROSCOPY

Results from 13 pointing observations of Hercules X-1 by HEAO 1 in the energy range 12-180 keV at different phases of the 35d on-off cycle for the duration of 1-10 hr are reported. A spectral deviation from the continuum at 30-50 keV, which is explained as a cyclotron line due to a transition between the quantized electronic states under the strong magnetic field at the surface of the neutron star, has been studied in detail. The pulse phase-averaged spectrum does not show any apparent variation, except for the overall intensity, during the main-on state. The pulse phase-resolved spectrum - in 10 unequally divided phase bins - on the other hand, shows a moderately strong phase dependence. The cyclotron feature and the continuum parameter of the spectrum, which may be an indication of the plasma temperature of the emission area, have continuous variations which peak at the pulse maximum. The controversy of the nature of the line-an emission and/or an absorption process - is also studied. An absorption line has been found to be in favor, at least during the main pulse, for its lower χ2 value in the spectral fitting procedure and energy dependence of the pulse fraction. Thus, certain geometrical structures of the emission region are more favorable than the others.