In Table 1, we list some information on the 13 luminous globular cluster X-ray binaries in the Galaxy. A comprehensive study of the X-ray spectra of these luminous sources was made by Sidoli et al. (2001), who used the spectral range of BeppoSAX observations between 0.1 and 100 keV. They find that the luminous sources in NGC1851, NGC6712 and NGC6624 have similar spectra. When a two-component model (the sum of a disk-blackbody and a Comptonized spectrum) is used to describe the spectrum, the fitted radii and temperatures are compatible with values expected for radii and temperatures of the inner disk. The spectrum of the luminous source in NGC6652 is similar, except that some radiation is blocked, possibly by the outer disk (Parmar et al. 2001). On the other hand, the spectra of the luminous sources in NGC6440, NGC6441, Terzan2 and Terzan6 are very different. In the two-component model the inner disk temperature was higher than that of the seed spectrum injected into the Comptonizing plasma, and the inner radius was smaller than those of realistic neutron-star radii. BeppoSAX observed the Rapid Burster in Liller1 and the luminous source in Ter1 when these sources were in a low state; the two luminous sources in NGC7078 could not be resolved.
Sidoli et al. (2001) suggest, on the basis of binary systems whose orbital periods are known (see Table 1), that the two types of spectra correspond to two types of orbital periods: the ultrashort-period systems (observed in NGC6712 and NGC6624) and the longer/normal period systems (observed in NGC6441 and NGC7078-1). We classify the sources as ultrashort or normal based on this correspondence, in Table 1, column (9). It may be noted that this classification does not depend on the physical interpretation of the spectra. Terzan 5 has been added to the suggested ultrashort-period systems, on the basis of its X-ray spectrum as observed with Chandra (Heinke et al. 2003a).
It is interesting to compare this tentative classification with two others. The first of these is based on the finding that ultrashort-period systems have a much lower ratio of optical to X-ray flux than systems with longer periods: the optical flux is due to reprocessing of X-rays in the accretion disk, and a small accretion disk thus has a small optical flux (Van Paradijs & McClintock 1994). Thus the absolute visual magnitude, in conjunction with the X-ray luminosity, may be used to estimate whether the orbital period is ultrashort or not. This is done in column (8) of Table 1. The other tentative classification scheme is based on the notion that the white dwarf donor stars in ultrashort-period systems do not contain hydrogen. The X-ray bursts of hydrogen-free matter can reach higher luminosities because the Eddington limit is higher in the absence of hydrogen. Kuulkers et al. (2003) have carefully investigated the maximum observed luminosities of bursters in globular clusters. On this basis we can also tentatively classify ultrashort-period systems, as we have done in column(10) of Table 1.
It is seen that the different classifications are consistent for known ultrashort-period systems in NGC6624 and NGC6712 and tentatively classified ultrashort systems in NGC1851 and NGC6652 and for the systems with known longer period in NGC6441 and Terzan 6. Two tentative indicators for the source in Terzan 2 are contradictory.
Five of the thirteen luminous X-ray sources are transients. The source in Ter1 has been consistently luminous until about 1999, when it switched off (Guainazzi et al. 1999). The Rapid Burster in Liller1 and the luminous source in Ter6 are recurrent transients, showing outbursts quite frequently. Intervals of ~ 6-8 months (Lewin et al. 1995) and ~ 100 days (Masetti, 2002) were observed for the Rapid Burster, and ~ 4.5 months for the luminous source in Ter6 (in 't Zand et al. 2003). The luminous source in NGC6440 is a transient whose rare outbursts have been detected in 1971, 1998 and 2001 (see section 2.1). The transient source in Ter5 entered a rare high state in August 2000 (Heinke et al. 2003a, and references therein). Interestingly, most (known and suggested) ultrashort-period systems are persistent sources. (The source in NGC6652 does occasionally drop below ~ 1036 erg s-1, but it is not known by how much.) Whether the above correlations are significant remains to be seen, and it will only become evident once more secure orbital periods have been determined.
With Chandra, the positions of the luminous sources have become more accurate. In Figure 2 we show these positions, together with those of the low-luminosity sources that also contain a neutron star. It is seen that some sources, e.g. the luminous source in NGC6652, are at a large distance from the cluster core.
Figure 2. Distance of Low-Mass X-ray Binaries to the center of the globular cluster in which they are located, in units of the core radius rc. Luminous and low-luminosity sources are indicated with and o, respectively. Errors are computed from the uncertainty in the X-ray position and from the uncertainty in the position of the cluster center (assumed to be 1.2"). Core radii and centers are taken from Harris (1996, February 2003 version), except for Terzan 6 (in 't Zand et al. 2003). References for the X-ray positions are in Tables 1 and 2. It is seen that most, but not all, X-ray binaries are within 2rc.