BeppoSAX study of the broad-band properties of luminous globular cluster X-ray sources

We have performed a detailed study of the broadband spectra of the luminous (greater than or similar to 10(36) erg s(-1)) globular cluster X-ray sourc es using BeppoSAX. With the exception of X 2127+119, located in NGC 7078, a ll the other spectra are well represented by a two component model consisti ng of a disk-blackbody and Comptonized emission. The measured low-energy ab sorptions are in good agreement with those predicted from optical measureme nts of the host globular clusters. This implies that there is little intrin sic X-ray absorption within the binaries themselves, and that the above spe ctral model provides a good representation of tile low-energy continua. The sources can be divided into two groups. In the first group, composed of 3 ultra-compact (orbital period <1 hr) sources, the disk-blackbody temperatur es and inner-radii appear physically realistic and the Comptonization seed photons temperatures and radii of the emission areas are consistent with th e disk temperatures and inner radii. For all the other sources, the disk-bl ackbody parameters appear not to be physically realistic and the Comptoniza tion parameters are unrelated to those of the disk-blackbody emission. If t his is a spectral signature of ultra-compact binaries, this implies that no other ultra-compact binaries are present among those studied here. It is u nclear why this difference between the two types of binaries should exist. One possibility may be related to the mass ratio, which is similar in the u ltracompact systems and binaries containing black holes. In the latter syst ems the soft components are also well-fit with disk-blackbody models, which appear to have physically realistic parameters.