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. |
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LMXB Name
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Cluster
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Ref. | |||
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(hr)
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X0512 | -401 | NGC 1851 | 1. | 9 | ![]() |
85 | [37![]() ![]() |
X1724 | -307![]() |
Terzan 2 | 4. | 3 | .. | . | [37![]() ![]() |
X1730 | -335 | Liller 1 | 2. | 2 | .. | . | [37![]() ![]() |
X1732 | -304 | Terzan 1 | 0. | 5 | .. | . | [37![]() ![]() |
X1745 | -203 | NGC 6440 | 0. | 9 | .. | . | [37![]() ![]() |
X1745 | -248 | Terzan 5 | .. | . | .. | . | [37![]() |
X1746 | -370 | NGC 6441 | 7. | 6 | 5. | 70 | [37![]() ![]() ![]() |
X1747 | -313 | Terzan 6 | 3. | 4 | 12. | 36 | [37![]() ![]() ![]() |
X1820 | -303 | NGC 6624 | 40. | 6 | 0. | 19 | [37![]() ![]() ![]() |
X1832 | -330 | NGC 6652 | 2. | 2 | 0. | 73 | [37![]() ![]() |
X1850 | -087 | NGC 6712 | 0. | 8 | 0. | 33 | [37![]() ![]() ![]() |
X2127 | +119 | NGC 7078 | 3. | 5 | 17. | 10 | [37![]() ![]() |
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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 [37, 73], X1850-087 in
NGC 6712 [28, 12, 115], 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 [64] 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.