4.1. Low-luminosity low-mass X-ray binaries

We consider a low-luminosity low-mass X-ray binary with a neutron star, LMXB^NS, securely classified when its luminosity is high enough (L_X 10³² erg s^-1, say) and its X-ray spectrum is soft (black body color temperature about 0.1 to 0.3 keV). The reason for this is that most soft X-ray transients in the galactic disk have these properties when they contain a neutron star. Their quiescent X-ray spectra have been roughly described as Planck spectra with a temperature of about 0.3keV (Verbunt et al. 1994), but more correctly should be fitted with model spectra of neutron star atmospheres as have been computed by e.g. Rajagopal & Romani (1996) and Zavlin et al. (1996). For quiescent transients in the disk, such fits give effective temperatures of 0.1-0.2keV and neutron star radii of roughly 10km (Rutledge et al. 1999).

The situation is more problematic if a transient in quiescence has a power-law spectrum and a luminosity in the range 10³¹ -10³⁴ erg s^-1. In that case, the system could be either a LMXB^NS or a low-mass X-ray binary with a black hole, LMXB^BH (see Tomsick et al. 2003, Wijnands et al. 2003). A hard spectrum can also indicate a cataclysmic variable, as may be the case for one or two sources in NGC6652 and Terzan1.

Figure 9. X-ray spectra as observed (i.e. not corrected for interstellar absorption) of low-luminosity X-ray binaries with neutron stars, as observed with ROSAT and XMM in NGC6205 (M28) (Gendre et al. 2003b; Verbunt 2001); and with Chandra in NGC6397 (shifted by +1; in 't Zand, private communication; Grindlay et al. 2001b). The solid lines show fits with models for hydrogen atmospheres of neutron stars.

The Chandra ACIS and XMM instruments are sensitive enough to detect these luminosities (L_x 10³² ergs^-1) in any cluster that they observe, with sufficient counts to determine whether the spectra are power laws or thermal (i.e., soft). The Chandra HRC is also sensitive enough, but does not have much spectral resolution. Sources for which fits with neutron star atmosphere models have been shown to give a good description of the X-ray spectrum include X7 in Cen (Rutledge et al. 2002, see also Gendre et al. 2003a), X5 and X7 in 47Tuc (Heinke et al. 2003b), B in NGC6397 (Grindlay et al. 2001b), CX1 in NGC6440 (in 't Zand et al. 2001), and Ga in NGC6205 (M13, Gendre et al. 2003b).

Most of these sources were already detected with ROSAT, being (among) the most luminous sources in each cluster (the exception is CX1 in NGC6440). As noted above, CX1 in NGC6440 is the transient, detected in the bright state in 1998 and 2001; whether the transient of 1971 was the same source cannot be ascertained. This source confirms the conclusion that the more luminous (L_x 10³² erg s^-1) among the low-luminosity soft sources are quiescent accreting neutron stars.

Probable classifications as low-luminosity LMXB^NS, based on the ratio of soft to hard counts as detected with Chandra have been suggested for 4 of the most luminous faint sources in NGC6440 (Pooley et al. 2002b), and in Terzan5 (Heinke et al. 2003a). Further probable identifications are based on the luminosity of the sources: 3 low-luminosity LMXB^NS (in addition to the Rapid Burster) in Liller1 (Homer et al. 2001b), 1 or 2 in NGC6652 (Heinke et al. 2001). We want to point out, however, that it cannot be excluded that some of these are LMXB^BH (see above).

A low-mass X-ray binary with a black hole can have a much lower luminosity than a LMXB^NS; as an example, for the transient A0620-00 in quiescence L_X 10³⁰ erg s^-1, much of which could even be due to the donor in the binary (Verbunt 1996, Bildsten & Rutledge 2000). At such low luminosities, even Chandra or XMM observations cannot provide a secure classification, and consequently we have no information of the number of low-luminosity low-mass X-ray binaries with a black hole accretor.

So far, only two low-luminosity LMXB^NSs in globular clusters have been identified optically, one in 47Tuc and one in Cen (Edmonds et al. 2002b, Haggard et al. 2003).