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

TL;DR

This study analyzes broadband X-ray spectra of accreting neutron stars with Suzaku, revealing a bi-modal cutoff energy distribution unrelated to system type, and exploring correlations with cyclotron lines, luminosity, and iron line features.

Contribution

It uncovers a bi-modal cutoff energy distribution in neutron star spectra and examines its relation to cyclotron lines and accretion properties, providing new insights into neutron star emission mechanisms.

Findings

Bi-modal cutoff energy distribution independent of system type.

Saturation of cutoff energies at high cyclotron line energies.

Smaller iron Kα line EW in SFXTs compared to classical HMXBs.

Abstract

We present a broadband spectral analysis of accreting neutron stars using data from XIS and PIN onboard \emph{Suzaku}. From spectral fits of these sources with a single continuum model including a powerlaw 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 cutoff energy. Among these 29 sources, 18 have cutoff 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 broadband coverage of \emph{Suzaku} also shows that the cutoff 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$\alpha$ lines. Finally, we show that the EW and flux of the iron K$\alpha$ line are smaller in SFXTs than classical NS-HMXBs. We discuss these findings in terms of different properties of stellar winds and accretion mechanisms.