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

Abstract

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 30.5ks and 34.4ks, respectively. In the 2005 and 2006 observations, the source was detected at an average 10--30keV intensity of 290mCrab and 230mCrab, respectively. The intrinsic pulse period was measured on both occasions at 1.23776s by HXD-PIN, after barycentric and binary corrections. The pulse phase-averaged spectra in the energy range above 10keV were well fitted by the ``Negative and Positive power-law times EXponential'' (NPEX) model, multiplied by a fundamental cyclotron resonance scattering feature at ∼36keV, which appears very significantly in the HXD-PIN data. The resonance profiles were successfully reproduced by a Lorentzian-type scattering cross section, rather than by a Gaussian-type alternative. The pulse phase-averaged HXD-GSO data, covering 50-120keV, 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 ∼73keV, with a depth of 1.6_-0.7^+0.9. This makes Her X-1 a 6th pulsar with established second-harmonic resonance. The implications of these results are briefly discussed.