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3.2 Low-mass X-ray binaries

The X-ray luminosities of low-mass X-ray binaries are in the range LX ∼ 1036 –1038 erg∕s. The upper limit is close to the Eddington limit for accretion onto a neutron star, so these systems must contain an accreting neutron star or black hole. All of the LMXBs in globular clusters contain an accreting neutron star as they also exhibit X-ray bursts, indicating thermonuclear flashes on the surface of the neutron star [124Jump To The Next Citation Point]. Compared with ∼ 100 such systems in the galaxy, there are 13 LMXBs known in globular clusters. The globular cluster system contains roughly 0.1% of the mass of the galaxy and roughly 10% of the LMXBs. Thus, LMXBs are substantially over-represented in globular clusters.

Because these systems are so bright in X-rays, the globular cluster population is completely known – we expect no new LMXBs to be discovered in the globular cluster system (unless more multiple sources are resolved from these 13 sources). The 13 sources are in 12 separate clusters. Three have orbital periods greater than a few hours, four ultracompact systems have measured orbital periods less than 1 hour, and six have undetermined orbital periods. The period of X1746-370 in NGC 6441 has recently been measured at Porb = 5.16 h using the Rossi X-ray Timing Explorer (RXTE) [12]. A member of the ultracompact group, 4U 1820-30 (X1820-303) in the globular cluster NGC 6624, has an orbital period of 11 minutes [225]. This is the shortest known orbital period of any binary and most certainly indicates a degenerate companion. The orbital period, X-ray luminosity, and host globular clusters for these systems are given in Table 1.

The improved resolution of Chandra allows for the possibility of identifying optical counterparts to LMXBs. If an optical counterpart can be found, a number of additional properties and constraints for these objects can be determined through observations in other wavelengths. In particular, the orbital parameters and the nature of the secondary can be determined. So far, optical counterparts have been found for X0512–401 in NGC 1851 [109], X1745–203 in NGC 6440 [235], X1746–370 in NGC 6441 [47], X1830–303 in NGC 6624 [131], X1832–330 in NGC 6652 [48Jump To The Next Citation Point99Jump To The Next Citation Point], X1850–087 in NGC 6712 [3811169], X1745-248 in Terzan 5 [100Jump To The Next Citation Point], and both LMXBs in NGC 7078 [8239Jump To The Next Citation Point]. Continued X-ray observations will also further elucidate the nature of these systems [165].


Table 1: Low-mass X-ray binaries in globular clusters: Host clusters and LMXB properties.










LMXB Name
Cluster
LX
Porb
Ref.
(×1036 erg∕s)
(hr)





X0512–401 NGC 1851 1.9 < 0.85 [48Jump To The Next Citation Point217Jump To The Next Citation Point]
X1724–3071 Terzan 2 4.3 [48Jump To The Next Citation Point217Jump To The Next Citation Point]
X1730–335 Liller 1 2.2 [48Jump To The Next Citation Point217Jump To The Next Citation Point]
X1732–304 Terzan 1 0.5 [48Jump To The Next Citation Point217Jump To The Next Citation Point]
X1745–203 NGC 6440 0.9 [48Jump To The Next Citation Point217Jump To The Next Citation Point]
X1745–248 Terzan 5 [48Jump To The Next Citation Point]
X1746–370 NGC 6441 7.6 5.70 [48Jump To The Next Citation Point181Jump To The Next Citation Point217Jump To The Next Citation Point]
X1747–313 Terzan 6 3.4 12.36 [48Jump To The Next Citation Point181Jump To The Next Citation Point217Jump To The Next Citation Point]
X1820–303 NGC 6624 40.6 0.19 [48Jump To The Next Citation Point181Jump To The Next Citation Point217Jump To The Next Citation Point]
X1832–330 NGC 6652 2.2 0.73 [48Jump To The Next Citation Point181Jump To The Next Citation Point]
X1850–087 NGC 6712 0.8 0.33 [48Jump To The Next Citation Point181Jump To The Next Citation Point217Jump To The Next Citation Point]
X2127+119-1 NGC 7078 3.5 17.10 [48Jump To The Next Citation Point181Jump To The Next Citation Point217]
X2127+119-2 NGC 7078 [48Jump To The Next Citation Point181Jump To The Next Citation Point239]










 

The 13 bright LMXBs are thought to be active members of a larger population of lower luminosity quiescent low mass X-ray binaries (qLMXBs) [240Jump To The Next Citation Point]. Early searches performed with ROSAT data (which had a detection limit of 1031 erg∕s) revealed roughly 30 sources in 19 globular clusters [124]. A more recent census of the ROSAT low luminosity X-ray sources, published by Verbunt [233], lists 26 such sources that are probably related to globular clusters. Recent observations with the improved angular resolution of Chandra have begun to uncover numerous low luminosity X-ray candidates for CVs [888999110100101545575182183]. For a reasonably complete discussion of recent observations of qLMXBs in globular clusters, see Verbunt and Lewin [234Jump To The Next Citation Point] or Webb and Barret [237] and references therein.


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