Neutron Stars and Black Holes in X-Ray Binaries

Abstract

The status of observational research into the properties of X-ray binaries is briefly reviewed, with emphasis on the similarities and differences between systems containing accreting neutron stars and those that likely contain an accreting black hole. X-ray spectroscopic as well as X-ray timing differences exist between these two groups, but there is no hard evidence for the existence of any “black hole signature” that by itself alone allows identification of an accreting black hole in an X-ray binary. The similarities in the properties between the two groups might be as interesting as their differences. If these systems really contain neutron stars and black holes, respectively, they can be expected to have strong qualitative as well as quantitative differences, such as, e.g., the presence/absence of a surface and differences in mass or magnetic field strength. By studying similar phenomena in such different systems knowledge can be gained on both the nature of the studied phenomena and the physical differences between the accreting compact objects. It seems likely that the X-ray spectra of accreting low magnetic-field neutron stars can, when they are faint, become as hard as those of black-hole candidates in the 1–30 keV and 10–100 keV, perhaps even in the 20–500 keV ranges. There are also striking similarities in some of the timing properties of neutron stars and black-hole candidates. A possible synthesis is discussed of the rapid X-ray variability properties of high and low magnetic-field neutron stars and black-hole candidates.