Comprehensive broad-band study of accreting neutron stars with Suzaku: Is there a bi-modality in the X-ray spectrum?

Abstract

ABSTRACT We present a broad-band spectral analysis of accreting neutron stars using data from XIS and PIN onboard Suzaku. From spectral fits of these sources with a single continuum model including a power law and high energy cut-off, cyclotron lines (where required), we studied the correlation between various spectral parameters. Among 39 sources we studied, 16 are those where the existence of a cyclotron line is known in literature, and 29 need a cut-off energy. Among these 29 sources, 18 have cut-off energy bunched in a range of 3–10 keV while for 11 sources, it spreads over 12–25 keV. This bi-modal behaviour is not based on the specific nature of the systems being a Be XRB or supergiant HMXB, nor on different beaming patterns characterizing their X-ray emission (as inferred from simultaneous study of their pulse profiles). The broad-band coverage of Suzaku also shows that the cut-off energies saturate for higher values of cyclotron line energies – consistent with previous works in literature – for both the groups and the width of the cyclotron line show a weak correlation with the cyclotron line energy. We also find an anticorrelation with luminosity for both spectral index and folding energy, respectively. Unlike previous works, we did not detect any anticorrelation between X-ray luminosity and EW of K α lines. Finally, we show that the EW and flux of the iron K α line are smaller in SFXTs than classical NS-HMXBs. We discuss these findings in terms of different properties of stellar winds and accretion mechanisms.