Analysis of Bright Source Hardness Ratios in the 4 yr Insight-HXMT Galactic Plane Scanning Survey Catalog

Abstract

We conduct a statistical analysis of the hardness ratio (HR) for bright sources in the 4 yr Galactic Plane Scanning Survey (GPSS) catalog of Insight-HXMT. Depending on the stable (variable) flux F s (F v) or spectrum S s (S v) of each source, the bright sources are classified into three groups: F v&S v, F v&S s, and F s&S s. Our study of the HR characteristics in different types of sources reveals that accretion-powered neutron star (NS) low-mass X-ray binaries (LMXBs) exhibit softer energy spectra than NS high-mass X-ray binaries (HMXBs), but harder energy spectra than black hole (BH) binaries in most cases. This difference is probably due to their different magnetic field strengths. Additionally, F v&S v LMXBs tend to be harder than F v&S s LMXBs below 7 keV, while the opposite is true for HMXBs. Our results suggest that LMXBs may dominate unclassified sources, and NS binaries are likely to be the primary type of X-ray binaries with ambiguous compact stars. By comparing the HR of transient sources in their outburst and low-flux states, it is found that the averaged HR of four sources in the two states are roughly comparable within uncertainties. We also investigate the spatial properties of the three groups and find that F v&S v sources are mainly located in the longitude of $-20^\circ<l<9^\circ$, F v&S s sources cross the Galactic Plane, and F s&S s sources are predominantly concentrated in 19°<l<42°. In addition, analyzing the HR spatial distributions shows the absorption of soft X-rays (primarily below 2 keV) in the Galactic Plane.