Spectral classification of low-mass X-ray binary (LMXB) energy spectra with color-color diagrams.

Abstract

In this article we describe spectral variations of a large sample of low-mass X-ray binaries (LMXBs). The observations are done with the European X-ray Observatory EXOSAT. We decomposed the measured X-ray spectrum into three energy bands and plotted them into color-color and hardness-intensity diagrams. Correlations are compared with a few template sources and then classified. With the help of a detailed spectral analysis of Cyg X-2 and Sco X-2 we also mathematically parameterized the color-color data of the bright sources. The colors of high luminosity LMXBs show significant intrinsic correlations which can be described in terms of three previously defined spectral modes, a 'horizontal', a 'normal', and a 'flaring' branch. Together these branches form a twisted Z in the color-color diagram. Four sources can be associated with the complete Z-shape. Three have been observed in two modes which always included the normal branch. Another three bright sources showed positive color correlations which could either be associated with normal or flaring branch behaviour. A spectral parametrisation demonstrated that the Z-shape of color-color variations in bright sources can be described by three parameters only. It also allowed a correction for the color-color data for interstellar absorption and showed that the parameter values are very similar from source to source. The persistent emission from the lower luminosity burst sources also show intrinsic spectral variations. Several different spectral states could be observed in different sources which seem to be related with each other, but with no evidence for the Z-shape in the color-color diagram found for the bright sources.