Suzaku Observations of Hercules X-1 : Measurements of the Two Cyclotron Harmonics

The accretion-powered pulsar Her X-1 was observed with Suzaku twice in its main-on state, on 2005 October 5-6 and 2006 March 29-30, for a net exposure of 30.5 ks and 34.4 ks, respectively. In the 2005 and 2006 observations, the source was detected at an average 10-30 keV intensity of 290 mCrab and 230 mCrab, respectively. The intrinsic pulse period was measured on both occasions at 1.23776 s by HXDPIN, after barycentric and binary corrections. The pulse phase-averaged spectra in the energy range above 10 keV are well fitted by “Negative and Positive power-law times EXponential (NPEX)” model, multiplied by a fundamental cyclotron resonance scattering feature at ∼36 keV which appears very significantly in the HXD-PIN data. The resonance profiles were reproduced successfully by the Lorentzian type scattering cross section, rather than by a Gaussian type alternative. The pulse phase-averaged HXD-GSO data, covering 50-120 keV, are featureless. However, in a differential spectrum between the pulse-decay phase and off-pulse phase, the second harmonic cyclotron resonance was detected in the GSO data at ∼73 keV, with a depth of 1.6 −0.7. This makes Her X-1 a 6th pulsar with established second harmonic resonance. Implications of these results are briefly discussed.