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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 [87]. 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. A member of the ultracompact group, 4U 1820-30 (X1820-303) in the globular cluster NGC 6624, has an orbital period of 11 minutes [154]. 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.

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








LMXB Name
Cluster
Lx
Porb
Ref.
(× 1036erg/s)
(hr)








X0512 -401 NGC 1851 1. 9 < 0. 85 [37Jump To The Next Citation Point149Jump To The Next Citation Point]
X1724 -307View Footnote Terzan 2 4. 3 .. . [37Jump To The Next Citation Point149Jump To The Next Citation Point]
X1730 -335 Liller 1 2. 2 .. . [37Jump To The Next Citation Point149Jump To The Next Citation Point]
X1732 -304 Terzan 1 0. 5 .. . [37Jump To The Next Citation Point149Jump To The Next Citation Point]
X1745 -203 NGC 6440 0. 9 .. . [37Jump To The Next Citation Point149Jump To The Next Citation Point]
X1745 -248 Terzan 5 .. . .. . [37Jump To The Next Citation Point]
X1746 -370 NGC 6441 7. 6 5. 70 [37Jump To The Next Citation Point123Jump To The Next Citation Point149Jump To The Next Citation Point]
X1747 -313 Terzan 6 3. 4 12. 36 [37Jump To The Next Citation Point123Jump To The Next Citation Point149Jump To The Next Citation Point]
X1820 -303 NGC 6624 40. 6 0. 19 [37Jump To The Next Citation Point123Jump To The Next Citation Point149Jump To The Next Citation Point]
X1832 -330 NGC 6652 2. 2 0. 73 [37Jump To The Next Citation Point123Jump To The Next Citation Point]
X1850 -087 NGC 6712 0. 8 0. 33 [37Jump To The Next Citation Point123Jump To The Next Citation Point149Jump To The Next Citation Point]
X2127 +119 NGC 7078 3. 5 17. 10 [37Jump To The Next Citation Point123Jump To The Next Citation Point149]








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 [78], X1745-203 in NGC 6440 [160], X1746-370 in NGC 6441 [36], X1830-303 in NGC 6624 [92], X1832-330 in NGC 6652 [37Jump To The Next Citation Point73], X1850-087 in NGC 6712 [2812115], and X2127+199 in NGC 7078 [9]. Continued X-ray observations will also further elucidate the nature of these systems [111].

Recent observations with Chandra of 47 Tuc [64Jump To The Next Citation Point] and NGC 6397 [65] have revealed quiescent LMXBs which also harbor neutron stars in binaries and are detected by the thermal emission from the neutron star. Work is ongoing, and new developments can be expected in the next few years.



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