2.1. Notes on individual sources

NGC1851. The accurate Chandra position for the luminous source in NGC1851 confirms the previously suggested optical counterpart; this star is very faint, considering the brightness of the X-ray source, which suggests that the binary is an ultra-short period binary (see section 2 and Table 1), i.e. P_b < 1h (Homer et al. 2001a).

NGC6440. The luminous source in NGC6440 is a transient; outbursts were detected in 1971 with OSO-7 and Uhuru (Markert et al. 1975, Forman et al. 1976), and again in 1998 and 2001 with BeppoSAX (in 't Zand et al. 1999, 2001). The 1998 outburst was followed up with NTT and VLT observations. An optical transient was found at the approximate location of the X-ray transient (Verbunt et al. 2000). The 2001 outburst was observed with Chandra (in 't Zand et al. 2001), and the source was identified with one of four low-luminosity sources found earlier by Pooley et al. (2002b). The 1998 optical and the 2001 X-ray transient are the same source.

NGC6624. The luminous source in NGC6624 has an orbital period of 865 s, indicating that the donor is a white dwarf (Verbunt 1987). For such a donor, theory predicts that the orbital period increases with time: _b / P_b > 8.8 × 10^-8 yr^-1. However, observations made in the period 1967 to 1997 show a decrease in the period, of order _b / P_b = - 5.3 × 10^-8 yr^-1 (Van der Klis et al. 1993, Chou & Grindlay 2001). This continued decrease cannot be explained by changes in the disk size. However, the X-ray source is located close to the center of the cluster (King et al. 1993), and if the central density is high enough, acceleration of the binary in the cluster potential may explain the difference (Chou & Grindlay 2001). Further study is required as discrepancies exist between reported positions for the cluster's center, it is also important that the central density of the cluster be determined more accurately. A viable alternative may be that the donor is not a white dwarf, but a stripped core of a slightly evolved main-sequence star (Podsiadlowski, Rappaport & Pfahl 2002).

NGC6652. Chandra observations of NGC6652 show three low-luminosity sources in addition to the luminous source. The optical counterpart previously suggested (Deutsch et al. 2000) for the luminous source turns out to be one of the low-luminosity sources instead (Heinke et al. 2001). The Chandra data were obtained with the HRC and thus do not contain much spectral information. The visual brightness of the new optical counterpart of the luminous X-ray source is still very low; and the suggestion (Deutsch et al. 2000) that this source is an ultra-short period binary stands (see Table 1).

NGC7078. A Chandra observation of NGC7078 (M15) showed that this cluster contains two luminous sources, at a separation of 3", seen as a single source in earlier observations with instruments that have less spatial resolution (White & Angelini 2001). The presence of two sources actually had been predicted by Grindlay (1992), as a solution to a puzzle posed by previous observations. The high optical to X-ray flux ratio indicated that the central X-ray source is hidden by the accretion disk, and that only X-rays scattered in our direction by a corona are detected; this implies that the intrinsic X-ray luminosity exceeds the observed luminosity by almost two orders of magnitude (Aurière et al. 1984). However, burst observations indicated that the bursts reached the Eddington limit for the distance to M15; this implied that there was no blockage of radiation, and thus that the observed persistent flux was representative for the full luminosity (Dotani et al. 1990). The brightest of the two (7078-2, see Table 1) is the burster; the optical counterpart is probably a blue star with U = 18.6; its position is determined most accurately from its radio counterpart (Kulkarni et al. 1990). The less luminous source 7078-1 has the disk corona, and is identified optically with a 17.1 hr partially eclipsing binary (Ilovaisky et al. 1993). Its optical brightness and the orbital period - revealed by variable, non-total eclipses - indicate that the donor in this system is a sub-giant. Ultraviolet lines with strong P Cygni profiles indicate extensive mass loss from the binary. An analysis of the eclipse timing puts a rough upper limit on the period change of 0.01 d in 22 yr (Naylor et al. 1992; Ioannou et al. 2003). An extreme ultraviolet flux has been detected from M15. It was believed to come from the X-ray binary AC211, the optical counterpart of 7078-1 (Callanan et al. 1999). We suggest that some UV may also come from 7078-2 which allows for a direct view to the center of the accretion disk.

Ter1. When Ter1 was observed with BeppoSAX in April 1999, the luminosity had dropped to about 2 × 10³³ erg s^-1, indicating that the luminous source in this cluster had gone into quiescence (Guainazzi et al. 1999). Accurate positions for the luminous source had been obtained with EXOSAT (8" accuracy, Parmar et al. 1989) and ROSAT (5" accuracy, Johnston et al. 1995); remarkably, the source detected with Chandra is not compatible with these positions (Wijnands et al. 2002). Probably, all observations of the bright state before 1995 refer to the same source, since the detected luminosities are all similar at, or just below, 10³⁶erg s^-1 (Skinner et al. 1987; Parmar et al. 1989, Verbunt et al. 1995, Johnston et al. 1995). This source was discovered in 1980 during observations with Hakucho; only two bursts were observed in one week. The upper limit to the persistent flux was ~ 10³⁶erg s^-1 (Makishima et al. 1981). It is not clear whether BeppoSAX detected the faint state of the luminous source, or the low-luminosity source found with Chandra.

Ter5. Observations of Ter5 with Chandra show nine sources in addition to the transient; four of these are probably low-luminosity LMXB^NS (Heinke et al. 2003a). A possible optical counterpart is a faint blue (in infrared colors) star, at M_J 1.7 when the X-ray source was faint. Heinke et al. (2003a) note that the X-ray spectrum when the source is luminous is like that of NGC6624 and NGC6712, and suggest that the source is an ultra-compact binary (see Table 1). If that is the case, its high optical flux is surprising. Wijnands et al. (2003) find that the spectrum in quiescence (near 10³³erg s^-1) is dominated by a hard power-law component.

Ter6. Extended studies of Ter6 with RXTE show that the transient X-ray source in this cluster has fairly frequent outbursts, on average every 140 days (in 't Zand et al. 2003). An X-ray position, derived from a Chandra observation, and an improved position for the center of the cluster, found with ESO NTT observations, show that the X-ray source is close to the cluster center. The RXTE observations provide an upper limit to the change in the orbital period: | / P| < 3 × 10^-8 yr^-1.

Liller1. The Rapid Burster in Liller1 is a recurrent transient. It shows a bewildering variety of X-ray behaviour. When discovered in 1976 (Lewin et al. 1976), it emitted X-rays largely in the form of very frequent bursts (which were later called type II bursts). The average burst rate was in excess of 10³ per day; this gave the source its name. There is an approximate linear relation between the burst fluence and the waiting time to the next burst (i.e. the mechanism is like that of a relaxation oscillator). These rapid bursts are the result of spasmodic accretion. Type II bursts have been observed that lasted up to ten minutes with a corresponding waiting time to the next burst of ~ 1 h. At times (early in an outburst which typically lasts several weeks), for periods of many days, the Rapid Burster behaved like a normal LMXB (i.e., persistent emission, but no type II bursts). The Rapid Burster also produces the thermonuclear, type I, bursts (Hoffman, Marshall & Lewin, 1978). A review of this remarkable source is given by Lewin et al. (1993). An accurate Chandra position of the Rapid Burster (Homer et al. 2001b) coincides with the radio counterpart (Moore et al. 2000). The Einstein position of the Rapid Burster (Hertz & Grindlay 1983) is not compatible with the radio counterpart and with the Chandra position. However, it does coincide with one of three low-luminosity sources also detected with Chandra. Perhaps the low-luminosity source was more luminous at the time of the Einstein observations. On the basis of their luminosities, the low-luminosity sources are probably low-mass X-ray binaries in quiescence (Homer et al. 2001b).