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

¹ Astrophysics Division, Space Science Department of ESA, ESTEC, Postbus 299, 2200 AG Noordwijk, The Netherlands
² Osservatorio Astronomico di Roma, Via Frascati 33, Monteporzio Catone, 00040 Roma, Italy
³ Astronomical Institute, Utrecht University, PO Box 80000, 3508 TA Utrecht, The Netherlands
⁴ Istituto Tecnologie e Studio Radiazioni Extraterrestri, CNR, Via Gobetti 101, 40129 Bologna, Italy

Corresponding author: L. Sidoli, lsidoli@astro.estec. esa.nl

Received: 16 October 2000
Accepted: 12 December 2000

Abstract

We have performed a detailed study of the broadband spectra of the luminous (≳ 10³⁶ erg s^-1) globular cluster X-ray sources using BeppoSAX. With the exception of X 2127+119, located in NGC 7078, all the other spectra are well represented by a two component model consisting of a disk-blackbody and Comptonized emission. The measured low-energy absorptions are in good agreement with those predicted from optical measurements of the host globular clusters. This implies that there is little intrinsic X-ray absorption within the binaries themselves, and that the above spectral model provides a good representation of the 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 temperatures and inner-radii appear physically realistic and the Comptonization seed photons temperatures and radii of the emission areas are consistent with the disk temperatures and inner radii. For all the other sources, the disk-blackbody parameters appear not to be physically realistic and the Comptonization parameters are unrelated to those of the disk-blackbody emission. If this is a spectral signature of ultra-compact binaries, this implies that no other ultra-compact binaries are present among those studied here. It is unclear 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 ultracompact systems and binaries containing black holes. In the latter systems the soft components are also well-fit with disk-blackbody models, which appear to have physically realistic parameters.