Observation of second cyclotron line feature in the spectrum of Her X-1

Abstract

Hard X-ray spectrum of Her X-1 was measured during a balloon flight experiment launched on March 30, 1997 using a large area scintillation counter telescope. The instrument code named LASE, operates in the 20-200 keV energy region and has high spectral and temporal sensitivity for the study of X-ray pulsators. The source observations were made on the 7th day of the 35d on cycle and at the binary orbital phase of 0.5. The measured source flux in the 20-50 keV region was found to be lower by a factor of about 3, the continuum spectrum was found to be extremely hard ? Apart from the primary cyclotron feature at 48 keV, the present data provides a clear confirmation of the absorption line feature around 96 keV corresponding to the second harmonic. We discuss the implication of hard X-ray spectrum and the line energy in terms of the properties of the emission region.