A Suzaku View of Accretion-powered X-Ray Pulsar GX 1+4

Abstract

We present results obtained from a Suzaku observation of the accretion-powered X-ray pulsar GX 1+4. A broadband continuum spectrum of the pulsar was found to be better described by a simple model consisting of a blackbody component and an exponential cutoff power law than the previously used compTT continuum model. Though the pulse profile had a sharp dip in soft X-rays (<10 keV), phase-resolved spectroscopy confirmed that the dimming was not due to an increase in photoelectric absorption. Phase-sliced spectral analysis showed the presence of a significant spectral modulation beyond 10 keV except for the dip phase. A search for the presence of a cyclotron resonance scattering feature in the Suzaku spectra yielded a negative result. Iron K-shell (K{}_α and {{{K}}}_β ) emission lines from nearly neutral iron ions (<Fe III) were clearly detected in the source spectrum. A significant {{{K}}}_α emission line from almost neutral Ni atoms was detected for the first time in this source. We estimated an iron abundance of ∼80% of the solar value and an Ni/Fe abundance ratio of about two times the solar value. We searched for an iron Ly{}_α emission line and found a significant improvement in the spectral fitting by inclusion of this line. We found a clear intensity modulation of the iron {{{K}}}_α line with the pulse phase with an amplitude of 7%. This finding favored an inhomogeneous fluorescent region with a radius much smaller than the size (∼3 × 10¹² cm) estimated by an assumption of homogeneous matter.